At level 3 Shadow Warrior does 28% of the character's damage, while level 5 does 40%. Warriors tend to have skill points to spare, so if you find the spell useful, I don't see any reason not to max it (unless there are other skills you find more useful).
Personally, playing a warrior I found it easier to just run over and start whacking opponents directly.
The Locus Inn - Skill System (individual paths, and a skill table for each class)