Defenders should only be able to close gates, and attackers open them on siege maps. I cannot count the number of times a defending traitor coordinates with attackers to open a side or back gate early on, resulting in a flood of enemy at the flag while defenders are at the main points of defence, and the flag is lost. This really circumvents the strategic aspect of the match, and is no different in spirit than teamkilling.
I concede that on certain maps, such as Helms Deep, gate opening is adventagious in certain circumstances for defenders. However, in the majority of maps this is not the case, and if this is what it takes to prevent defending traitors to cicumvent honest competition by openning gates for the enemy, then I say so be it.