The enhance comes up when I have a board that can't do white TPAs, or can't do them easily. It's basically either a delay, which is useful, or a auto-TPA activation since it gives a similar boost to the same monsters (and Sun Quan himself, but he hits much less hard because he's not boosted by Ra's enhance and I don't tend to go out of my way to make blue TPAs). It's versatile. Isis is versatile too (and has better stats) but I can't imagine her skill's three levels of utility (healing, bind recovery, short-cooldown blank skill) coming up as often as Sun Quan's (defensive delay, prong replacer, delay as pseudo-haste).
But yeah, I'm talking specifically about challenge dungeons mostly. And she's of slightly more limited utility there, and I feel much more like I need damage help.