Attackers should not kill Defenders when Transition Mode is in effect.
In my WZ experience, it turns out that this strategy only works against a) bad players and/or b) players who aren't using classes. I strongly discourage it.
Decent players, if allowed to live, will hold a transition mode force field for up to 2 minutes; I do it all the time on my Scrapper because people just try to CC without actually killing me. That lets me restealth, drop a bombardment on one console, and then harass the other console. And that's even after every other ally on my side is dead on the transition side of the force field, let alone all the time wasted killing those allies.
Here is a list of classes that really need to be killed because a decent player will be able to stop a force field opening. Heck, even an average or subpar player can use these classes to really mess up your transition mode offense.
1) Scoundrels: Evasion --> Disappearing Act is a nightmare for attackers. So is Flashbang. So is Freighter Flyby. So is Thermal Grenade. So is Sleep Dart.
2) Shadows: Shroud (Especially 5second Shroud) + Speed + Force Cloak is also a nightmare. Mind Maze is also just as problematic as Sleep Dart (although it is less spammable)
3) Sages: Fadeout Speed removes all slows/snares and grants slow/snare immunity. Good Bubbling can keep a team locked down for 6 seconds on its own. Force Wave has a big range, doesn't have to be aimed, and takes everyone off the door.
4) Marauders (don't know the Pub ability names): Intimidating Roar + Force Camouflage + Charge can really keep the attackers tangled up.
Just trying to CC lock these guys is a great way to get your offense stuck for 2+ minutes on a bridge. Just focusing offensive resources into murdering them outright gets you the bridge in 30-45 seconds.
Ultimately, it's a gamble. If you kill them, you have a much better chance of taking the bridge in a much shorter time. But then you won't get your awesome transition push against understaffed defenders on a door; everyone will have just respawned. On the other hand, if you don't kill the defenders in transition mode, you might be able to make a nice transition attack past the bridge...or you might get tangled up and waste a ton of time. The gamble is still balanced on the side of the first option, just because if you took the first door you probably have the resources to take the second door given enough time. Gambling on the second option can really cut into that badly needed time.