I agree that I would only consider using a 2-3 if I knew the opposition was going to use an inside tactic (which most do, but still) because, well... that's what a 2-3 zone is meant to do; help stop an inside offense.
As for training SB, I know a lot of managers have different opinions about this, so it is truly hard to say. Obviously with ID training comes SB pops, and many times this will be enough to reach an appropriate level in this skill. Although I don't think spending a very long time training SB is good, I personally believe it should be trained a little bit (rather than just relying on cross training). Having a player or two with SB would always be a nice callup option. Also, I hear many managers say that SB drives the salary of a player up way too much, but I think this may be a bit unfair to say. SB is cheaper than every other inside skill, and I think that the people who say this basically just see it has a worthless skill. Sure ID is probably more important, but SB is a nice secondary that will improve a players inside defense and help decrease fouls.
Quite obviously though, if a manager didn't know whether to focus more on ID or SB (because they were new) I'd tell them ID :P