I think teamplay in Nexuiz should be done otherwise, but not by slowing down everyone (on the other hand, I don't think the slowdown in havoc physics is too harsh - it still feels right to me).
An idea might be however to make maps with more possible teamplay elements. Sniper posts (NOT TOO MANY OF THEM) are just one of many things mappers can do to improve teamplay. Switch-controlled doors can also help a lot (but not like on mikeeusa maps where the door opens slowly and the switch is close to the door - I rather think of a switch easily accessible to people inside the base, and having additional switches outside it that also can open the door. Of course, in such cases, alternate ways must be provided to enter the base, so a single player can do it TOO - but that way should really not be the fastest one. I think the map "switches" is very extreme, but shows some things that can be done to improve teamplay. Imagine for example four pathways between the bases, of which only two can be open at a time, and "control panels" (in fact, sniper towers) that can shoot a switch for a pathway, and that one will open and another one will automatically close. Then a cooperative team can always prepare the right pathways for the escaping flag carrier, and close the door behind him, but also hinder the enemy from running away.