I'm going to guess that spell speed isn't that useful as it stands, but it will become more relevant as better gear is introduced and the level cap is increased. It seems to be a stat with increasing returns the higher you get but as it stands we just can't get enough of it to make huge difference.

Perhaps someone with lv70 HQ crafted gear who has crammed as much as they can on in materia can give a better indication if it's even worth it at this point.