Xalek is actually a interesting companion. He has tanking abilities, but his DPS is just as good as Ashara's due to his unique Lightning attacks. Unlike many Tanks trying to DPS, he actually pulls it off marvelously. I use him with my Tankasin and he destroys mobs with ease.
I also like Xalek because he has a Grey alignment. On Corellia, I got many Light Side gains and Dark Side gains with him.