While I am a huge minis fan, I think whether you use them and a grid should depend entirely on if the encounter actually benefits from them. Back when I ran a celtic-themed campaign there were plenty of times when the characters were wandering through a forest, and I would roll for random encounters. If something did happen, I would have to stop the game, draw out a section of forest (trees included), have them place their characters, then monsters, then roll for initiative, then finally get to the actual encounter.
Frankly the whole setup process was anticlimactic and a bit tedious. Knowing where trees, hills, rocks, etc were located was good for forced movement powers, positioning, cover from ranged attacks, and more...but in hindsight I could have just as easily told them that ranged attacks are at a -2 and that their speed is halved due to all the brush and other obstacles, in essence "tagging" the environment as you would in FATE. Avoiding the map would also maintain momentum by telling them that they have just been ambushed by cockatrices, and that they should really roll for initiative.
Another instance where I can see avoiding the map would be handy is when you are trying to freak out your players and/or run sequential combat challenges. When I tried to run Expedition to Castle Ravenloft, there were too many instances where the players would move, get into a fight, move, get into a fight, etc, so I spent a lot of time wiping down the map, drawing new terrain, and rolling new initiatives. Again, it got tedious, and the map gave them a good indication of where everything was.
Had I avoided using the map I felt that I could have immersed them more in the horror, not only because I would not have had to stop to draw things out, but the lack of map would have made them unsure of their surroundings (as well as allowing me to "cheat" if they tried to flee). Also not using exact minis meant that I could have described a monster and allowed them to mentally fill in the details on their own.
Where I think minis and maps can be really shine is for battles with varied terrain and things to manipulate. For example most of Keep on the Shadowfell did not really need a map. You could easily used abstracted movement to allow players to take a move action to close the distance with the kobolds, hide behind terrain, or just get really far away, and then just gone from there. Now the last battle with Kalarel? You had a blood pool, portal, stairs, pillars, etc to fiddle with, as well as a variety of monsters, so I think it was more worth it to use a map.
I have been using this approach in my current campaign to great effect, so am hoping that based on the poll results that WotC creates some decent rules for abstracting gridless combat.