Redmage still gets benefit from spellspeed. You have to hardcast to get dual cast procs, and that limits your resource generation. However, it doesn't directly benefit the TP skills, and may be less desirable than direct hit and crit due to this. Just like any other class, you will have to choose between I'll and stats if you want to min/max.

It's not a dead stat, it's just is lower than a lot of others. You could argue that it should also affect skill speed so it's benefits are similar to other casters though.