You know I cannot think of a single column of Wanderings Monsters that I have liked, or at least not one that I have regarded in a mostly favorable light. I read them, wonder why they would try pitching something so bland, confusing, uninspired, and/or that panders to previous editions for no discernible reason, and just hope that the final draft is much, much better.
Frankly I think that the flavor I cook up as a reaction to these articles is better, and I am not a professional in this industry by any stretch. Not only have these guys been in the business for years, but they have a whole team working on it, so why are the results almost always so...bleh?
Part of the problem with summoning magic is the same problem that has plagued magic in general: it is too safe and predictable (and makes no sense).
Trying to summon something from the Nine Hells or Elemental Chaos could be a big deal--it depends on what you are going for--but really never is: just wait until you get to a certain level, take a certain spell, and voila; you will always be able to reliably summon the same creature in a couple seconds, it will always listen to you barring some specific corner-case ability or effect, and once the duration elapses it will disappear whence it came (again, barring some corner-case ability or effect).
I think that how summoning magic works should vary from class to class, and if I were in charge of design wizards would unfortunately get shafted because I see them needing to spend more time than most drawing a circle, preparing ritual components, tearing open the planar fabric so that they can either attract or draw a demon through, then either bargaining with or imposing their will upon it.
How long it takes and how hard it is to get the demon to do what you want would vary by the strength of the demon. It should be easy even for a relatively green wizard to conjure up and boss around a dretch, while a babau is going to take some convincing. I would also give a bonus for taking extra time, using exceptional materials, being skilled in certain schools of magic (like abjuration), or having people helping you out.
Conversely you could eschew these things or cut corners, but then you would take a penalty to conjuring and/or containment. If classes and subclasses were flexible and not small preset bundles then you could introduce features that made you better at it, granting bonuses so that you could take shortcuts while still retaining some measure of reliability. Of course if you could still go through the motions anyway, making it do more or getting something even better.
On the topic of sacrifices, I really do not like the notion that you have to give a demon something. For low-level lackeys it might be necessary, but what about a powerful conjurer who knows truenames and has plenty of experience bending reality? Is forcing a demon to obey her through sheer force of will any different than unleashing bolts of force, reducing a giant to a toad, controlling someone's mind, or crossing vast distances in the blink of an eye?
In my system sacrifices would not be mandatory, but could still be used to sweeten the deal (as could places of power and certain events). This is where you get groups of cultists all working together to get a powerful demon to do their bidding, often in a corrupted temple, possibly during an eclipse or at dusk, but a particularly skilled and powerful conjurer could still do it by herself sans offerings.
Finally I do not think that demon summoners and the act of summoning demons should be an intrinsically evil act. Most people probably do it for less than altruistic reasons, but you could still have exceptions, and I think that leaving those options open makes it easier on storytelling. Kind of like how some people think that all necromancy no matter what should be evil, but then you get "official" good-aligned totally-not-undead undead in Eberron. Just do us a favor and let us decide what, if anything, has to be absolutely evil.
Well that went on quite a bit longer than expected, so let us move on to gnolls.
The backstory for gnolls is that Yeenoghu gets summoned into the world, kills the people that summoned him for absolutely no reason, destroys a bunch of stuff, kills a bunch of people, and maybe gets beaten up by a halfling god. Some of the demonic hyenas get left behind and for some reason ignored, growing up to become gnolls. Also they somehow spread to other "known worlds" in the Dungeons & Dragons multiverse, because I guess Spelljammer was right all along.
Awhile back I mentioned that it is not always necessary to explain a monster's origins, and if this is the alternative I would rather have not known.
Gnolls could be a race of hyena-like humanoids that dwell in ruined cities or gather around ancient, bloody shrines dedicated to some sinister hyena god. If anyone gets too close they just kill and eat them, so no one has learned much about them, and after while I would hazard a guess that no one wants to. They would not necessarily be evil, just territorial, but still work as antagonists for basic dungeon- or hexcrawling. Simple and straightforward.
Now if you want them to all be evil you can do that too, in a much more interesting, visible way. What if gnolls ventured forth from their ruined cities to pillage, destroy, and capture prisoners. They eat some, but not all, forcing them to fight each other for survival in bloody arenas surrounded by grinning obelisks, watched over by demonic gnoll rulers. Unfortunately the "winners" of these bloody, brutal contests, those willing to do whatever it takes to survive, are transformed into gnolls. Think the Firefly episode Bushwhacked, just a much more overt physical change.
Not working for you? How about hyena-like demons that possess humans and transform them into gnolls? The gnolls gather the necessary sacrifices, open a gate to the abyss, and allow them to possess mortals. This way you get evil gnolls that you can kill without having to think about it too much, and even better you could mix up cults of Yeenoghu with hyena-like demons, gnolls, and un-transformed humans. Think about an isolated village where humans secretly eat travelers, wield flails or claw bracers, and are lead by gnolls.
Of course nothing says gnolls have to be evil. They might be strange looking, territorial, and worship one or more strange hyena gods (some good, some evil), but otherwise have a culture, history, and varying personalities and goals. This is what I would do in a campaign setting where I wanted to allow gnolls as characters.
I do not even know what to say about Orcus's story. It takes place in Forgotten Realms, which has become the Naruto of campaign settings because it will never end. Actually given that it just keeps getting changed over and over with each edition despite no one doing anything remotely interesting with it, a more appropriate comparison might be Nintendo, who refuses to invent new heroes within their existing properties and just continues to keep recycling everything, sometimes adding a new gimmick.
I would like to see an Orcus backstory divorced from a specific setting, or that at least takes place in an interesting one.