A few times now, I've been in a party where we've been left without a tank and waited endlessly for another to come, since there clearly aren't enough. Occasionally, though, instead of quitting, we've had the crazy idea of tanking with Titan. Nearly every time we have, we've been fairly successful and were able to complete it without one.
Currently, Titan isn't an ideal tank; but as a last resort, he can be okay. With slight increases to defense and ability to hold hate and some new commands to give more control over his actions, SMN could easily tank.
Of course, that doesn't mean SMN should only be a tanking class. Their main role should stay DPS, but having the option to tank would not only reduce wait times, but allow for some more diversity, allowing a magic job to fulfill a role that has traditionally been reserved for melee characters.
In case anyone is curious, here's Titan finishing Neverreap:
http://i.imgur.com/DlzvY7k.png
What do you guys think?