I find them to be critical. I've had knee problems for years and set out to make them strong by walking a lot and then doing some simple leg extensions (sittin in a chair and straightening my legs then bringing down again). I slowly started incorporating lunges and squats into my routine, only a few reps and then gradually increasing. I seem tremendous results.
The woman pictured in my avatar (no that's not me lol!) is Sylvia Weisenberg, my inspiration for a fit body. She has an exercise video called Tonique Fitness. I kid you not, it is a half hour of various lunges and squats. It is the most difficult fitness program I have ever followed. I started off huffing and puffing within the first 10 seconds of the routine. I'm doing better now but I still can't complete the full video and I have to take lots of breaks during. I know that sounds scary but you feel results immediately. It's great for legs and glutes, cardio, and balance. I stop 20 minutes in.
http://toniquefitness.com/dvd.php