Scott here serves as basically a fast way to "leash" Phoenix once she's been unleashed, especially if you have White-Hot Room out (which you should, bonus healing and bonus power counters are two things Jean wants desperately). This is useful if you really need to thwart, and once he's at 1hp you can kill him to probably unleash Phoenix again, which is cute. He's also just got a very solid stat spread.
That said, I end up spending him as a resource more often than not, especially once Phoenix is unleashed- there's just more powerful things you can do, and if you're on 3 power counters you're pretty much SOL. Not the best personal ally.