I think the power advantage you get with lipo would be of very little benefit to 75% of offroaders as it isn't how fast your car is but it's hgoing to be those pesky thumbs at the end of the day. Doughty proved this using my car in one leg of the a final at epr and still very much on the pace. I couldn't get the car any where near that fast around the track.
The main benefit as stan said, is a couple of packs for both classes and reuseable all day
. There has been alot of people this year who have had cells go down in packs and have not replaced them purely because lipo is the future and there is no point in replacing damaged nimh's.(myself included)
I'll be using lipo as much as i can over the winter and club racing because it saves all the charging,discharging and equalising routine that seems to take a lifetime.
As for car fitment, at the level of racing that we are talking about, i think people will do whatever they have to so as to make their cars lipo compatible(if they feel they are at a disadvantage using nimh). They will fit every car with very little mods to the chassis inc. the s4(see doddy's thread)