If the goal is victory for our cause, and not just victory for some people who find it convenient to claim to be cheerleaders for our cause, we need to be very careful that our tactics are not training up Stalins within our ranks.

Well said. Also, an additional benefit of rational discussion is that it promotes truthseeking - people may discover that the cause that they're supporting is not the cause that they should be supporting. Under a "win at all costs" paradigm, arguments against your position are enemy soldiers, so if you win, it'll be without seriously considering the arguments of the opposition. That increases the likelihood of you being wrong. If your goal is something beyond personal power - if it's something like "the correct thing should win and become dominant" and not "I, as I am now, should win and become dominant" - then honest discussion is even more useful.