51 dives......
I worked in sales and customer service for a bike company and this was one of my biggest pet peeves. It's a TWO YEAR warranty. Not an "X" number of dives warranty. Your number is subjective and is irrelevant to the warranty.
I had people who would say "I know it's out of warranty by a month, but I've only used it 4 times!", then I'd have people that say "I know it's out of warranty by a month, but I've only used it 4012094801824 times!". If it's out of warranty it's out of warranty and they don't actually owe you anything.
Part of the issue, is fairness too. If you replace one person's that's technically out of warranty, but they only used it 4 times, than you've got to replace the person's that's out of warranty, but used it 780382082342 times as well, since there's no written information limiting the amount of dives under warranty.
I guess, maybe I've just had good luck with Shearwater, but I think they're being pretty damn fair.