It's one of the documents CIC says you must have when you arrive in Canada.
http://www.cic.gc.ca/english/study/study-arriving.asp
This page also states: "You may not be allowed into Canada if any of your documents are missing or if any of the information on your application or letters of reference is incorrect."
So it's possible you may not be allowed into Canada without the letter - or they might let you in. Impossible to say. But you should avoid the risk and have the letter with you.