Biki lime does damage compared on the Level of your Attack/Str (such as other powerful attacks of lime).
So, even if now you find it Slow and Useless, it probably becomes a LOT more useful going on.
For Example, at level 50, when you trasform the space attack could do around, dunno, 200-300 damage?
Now, i think that you wouldn't find useless to can spam 300 damage AOE without losing mana, simply pressing Space.
It's not THE BEST trasformation but it sure does its part at the correct point.