I ride on platforms for my mtn bike, and half platform/half SPD for cross. You know, I've been biking daily (commuting, etc) for years and years, and although I use clipless on the road bike (which I don't ride in downtown), I never fell in love with them for any other application.

Part of this is because I have a bum knee that doesn't like a tough unclip (which can happen if there's gunk in your pedal). The other part is that (on the mtb, on technical stuff) I'm just so much more confident on platforms - there's a lot more cool stuff that I'll try with the stress of not being able to unclip removed.

Oh, and I've never slipped off a pedal - if you are used to clipless, you probably would, but when you're used to staying glued to the pedals, it just doesn't happen - so don't worry too much about that!

I just ran into another woman at a race who (although having clipless) spends a lot of time unclipped. So lay off the pressure on your girlfriend! She can have fun doing cross no matter what pedals she's running. Yeah, she may not win, but cyclocross is more about the awesome than about the medals anyway