It's very true that the time series Unemployment does not count involuntary part time workers. However, the time series Involuntary Part Time Workers does count people who want to work more hours but can't.

The BLS tracks both these numbers.

http://www.bls.gov/news.release/empsit.t05.htm

I think half the criticism of the unemployment number is simply because people wish one number could explain everything, but unemployment does not.