On my data center it just seems that ABC goes for the tanks and healers, and dps just do whatever they want.

Generally speaking, though, B team always enters entirely, and a few dps from both A and C join them.
There's really no reason not to go in. Even if your healer isn't joining(except for B healer, who is only necessary to heal B tank), you'll be fine because the persistent damage does like nothing.

Honestly, it's pointless debating what the best tactic is, because in the end, people will just do whatever they want, because they've been getting away with it ever since the fight became pure farm status for the entire population of the game.