All the channels are for ISO tracks in film. Basically, you have track 1 which is your mix track, everything else boom, lavs, go on their own channel. That's why you're starting to see 4+ tracks. The Deva 16 should be out by NAB, so you'll have a real 16 channel portable recorder out there too. For the taper/hobbyist, I don't see much past 6 being that useful. However if you are doing a lot of recording, then you might have individual feeds from the board and staged audience mics, in which case you'll need more than the 4 or 6 channels these offer.
Wayne