I'm in favor of allowing uncommunicated cooperation in no-comms games, on the grounds that:
1) The game is more fun that way
2) It doesn't negate the primary goal of no-comms (to remove the overbearing effects of diplomacy)
3) It's hard to define exactly what counts as "cooperation"

Reasonable people can disagree, certainly.

Lesson learned for me: any no-comms game should specify in the rules if cooperation of any sort is banned, or if only explicit communication is banned.