It could probably be useful if people played in the same place, only with their laptops instead of pen & paper, or perhaps if they played live games through skype or something (which is I think what they have in mind). But I found it really cumbersome in pbp, because:
Right now, I have a fairly streamlined process when it comes to creating maps. You export the screenshot from maptools, I upload it to an one shot free image-hosting thing, and the map is available like this, doners.
Instead, in Roll20, you have to log in every time to their map thingy. Which compared to just clicking a .jpg link, seems to take ages. It might be a small delay, but it is an annoying delay when you are used in not having it and I found myself actually discouraged from clicking my own map, bored to go through the mini-login process.
Another gripe is that I often like to check the map from mobile devices. Either because I am curious to see what a player did (when they bother to post, grumble) and to begin thinking what to post in my turn even if I am not home, or in order to quickly point out that an action is invalid so that the player might correct it and not have the day go to waste. With Roll20 that is of course (or was when I tried it) impossible.
Another gripe, is that I had a pretty huge map, but it always insisted on starting my view (or anyone's) on the top left corner. I hope they fixed that by now. (aka, after the login, you had to zoom/pan around in order to actually see your character, every time)
And finally another gripe was that it has no lighting system like maptools has which saps your ability to make sexy dark places slowly being revealed by torch light without actually having to count squares yourself to see if something ought to be visible or not. (again, at least when I tied it).