Sorry, but I do buy into the vulnerability bit. The shields fail all the time, and giving a tangible target for an enemy take out the command crew is absurd. Yes there are other people who can take over, but it'll take time for them to figure out who's in charge and where, and an over all loss of efficiency in a crisis. The bridge should've been right smack in the middle of the saucer where a single hull breach couldn't wipe it out.
Thankfully for the Federation, it's a law of the Star Trek universe that death is usually instantaneous to any enemy who gives the command "target their bridge."