There should be in the first place any skills at all which should be useable only in combat.._ its complete pointless skill design imo
What exactly would change and be different, than before, if we would have no skill, which work only while being in combat?

I say - absolutely nothing at all.

Certain skills could be ecentually maybe used in preparation of a battle, to start a battle with a certain effect on already instead of having it a few seconds later, but thats it