I see no issue with it. Some people I suppose that this is all they can manage to the best of ability, and I say draw and keep drawing no matter what it is. from a different view though, if you want to improve as an artist, you need to step outside of your comfort zones and really explore/ experiment/ practice.
Depending on my how much detail they put it into can make a difference. I do face-shots, but not to the extent of limiting my capabilities. I think doing both is best so you can find your strengths and improve on your weaknesses. Had you just do face-shots alone, is quite restricting though.
I don't think it's really "good" or "bad" persay. If the ONLY reason you're ONLY drawing face shots is because you're literally AFRAID to try and draw even so much as a waist up shot then I think it can be a problem. I personally draw face shots now and again to break out of my full body shots routine.