If doing single leg squats hurt your knee, it means you don't have enough core strength or that your legs aren't ready for this exercises, and/or that your techniques in squat is poor.
If you can do 300 body weight squats in good form (neutral spine and embracing your core and having good balance and proper lower body aligment), you should be able to do single leg squats. But much of it comes from proper core strength and flexibility.
Single leg squats works also your abs/lower back. It is a very functional lower body execise. In wing chun, we build our stance (ma) and kicking power from this kind of exercises, there is more but that is like top secret. lol.
Mr. Bao
"A gung fu man, then, should be soft-yet not yeilding; firm-yet not hard." Lee Jan Fan