3v3 is the smallest number of people that you can have a decent game of DTAS. The old DTAS restricted games to 3v3 for that very reason. Because of the relative size of Infiltration maps, if you want to have *any* chance at being stealthy, you want to minimize the possible number of contacts in each game. Thus, 3 on a side would satisfy this condition. So, instead of combat being a slugfest between two sides, it would become more of a stealthy search and destroy/hide and capture fight. Limiting to three people also makes it difficult to completely seal off any one area; there will be gaps that a quiet, careful team could sneak through. It makes the skill of deciding where to defend of the players even more important. It also means that you can't have half the team defending, and half the team scouting - you'll have to have everyone stick together if you want backup. Sure, you could let one of three go out scouting, but what happens if they need backup? They won't have any, unless you have one person guarding the flag...
Anyway, that's my opinion.