That is true. You only get to work 20 hrs a week. However, consider that you get a part time job working 20 hrs a week on the work permit ( while working another job -cash preferably), by the time you finish school you'll atleast have some professional experience to show to your would-be employer. This would definitely give you an edge over other would be candidates who have no professional experience. If you are someone who plans for the future, you should understand what I mean. 

As for companies refusing to hire you because of the expiration date of your workpermit, that would be considered inequality/bias and you should bring up the issue with the HR department of the company. A company is not allowed to refuse you a job wherein you are the best candidate on the basis of your workpermit expiration date.

I hope this answers your question.  So go ahead apply for that work permit!