No HDD spinning noise, less vibration in the case, especially for people with several drives, fast recording of unrendered video, faster possible rendering times then, too.
Just gotta think out of the box, no one is saying it's a cheap thing that everyone should do, though. :P
Even if I had the money, I don't think I'm ready to jump to all SSDs, but SSDs won't fail from power failures like HDDs will. Remember when my computer had power issues back in April and my hard drives died?
All except my SSD... and DVD drive. XD SSDs can gain and lose power in rapid succession with no real damage, mechanical hard drives, obviously, cannot.