According to this NHTSA webpage, it says:
Quote:
Crash data and law enforcement crash reports show that belts jam in crashes fewer than one percent of the time.

Considering the large number of crashes every year, that's not an insignificant number of jammed belts nationwide. Now, whether a jammed belt made any difference in survival...that's a separate question.

So, take your chance of being in a crash *and* having a belt jam *and* being in a situation where you must get out of the belt quickly....that's a pretty small number for most of us, I think.