Although you chances of exiting the aircraft in 90secs if it crashed in the middle of the Atlantic in the middle of the night are pretty small - the FAA has to pretend it's possible.
What? This requirement is actually quite important. Take a look at:
http://en.wikipedia.org/wiki/Air_France_Flight_358
I followed a few links and skimmed the full report, but couldn't find the time it took to evacuate. But it was under 90 seconds, even with several exits not usable. Since the aircraft was already burning before it came to a stop, any time savings was critical to ensuring survival. And everyone did survive.
So I think this regulation is important. If there was just one exit, and it was blocked, there would have been 300 people dead in a fire instead of 0.