I think as long as you've decided it's not for you, then so be it.  We're not your mom. We don't now your personal situation.
Personally, I don't sweat it.  I drive a bridge to/from work every day, only about 40 feet up.  I'll probably survive a crash just fine.  I'll get out.  I have a knife like you.  If not, well, I've never NOT seen a belt unbuckle, despite numerous car accidents that I've responded to.  Worst case scenario?  I pull out my car keys and use them, or the POS knife on them, to cut free.  
As for car crashes and fire, again, not a very realistic threat.  Most cars don't catch fire, blow up, etc.  
By all means, if you've got the cash and inclination, buy one. Me, I'd rather not.