For Black Mages it can be very useful. Looking at BiS training dummy parser comparisons weighted to Crit vs. Spell Speed, the Spell Speed build out-performs the Crit build slightly in the long run. Add in needing to move around a lot in a boss battle, and it edges even further ahead because it's easier to squeeze spells in between needing to move. That said, it's not a huge difference, so whichever way you like to build your stats is fine. If you like seeing big numbers, then Crit is more than acceptable to focus on.

Here's some graphs to show you the differences in builds over the course of 4 minutes, illustrating that Spell Speed provides more damage in the long run. http://www.reddit.com/r/ffxiv/commen..._vs_sspd_with/