If Rinoa hadn't been up to the task before her, Squall would have stepped in to keep her safe. Since thus far it hadn't proved necessary, he stuck to what he knew best. Oh, he could have just as easily have been casting spells alongside Rinoa, but that wasn't really the point of a Knight. A Knight was supposed to keep his Sorceress safe and he couldn't see any better way to do just that then by tackling the hordes head on.

The fact that Rinoa had cast Haste only made things easier, as he took full advantage of the increase in speed to cut even deeper swathes into the oncoming Lumens. As far as enemies went, they weren't exactly much of a challenge and he'd pretty much stopped registering what the various oncoming Lumens looked like, save to register that it was, in fact, actually a Lumen.

(Not that that was hard, of course, given the current way of things, but best to be safe, in this case.)