I typically have an 11 on my bike. Not really because I need it, but because its flat here, and I don't need big gears, so I put on a smaller cassette, either 11-21, or 11-23.
Tuesday night world championships where the leadout of the sprints start in the low to mid 30's it doesn't hurt to have an 11, although I wouldn't say I need it.
The other use of the 11, is for muscle tension workouts. On the trainer, or slight grades that pass for hills, I can use the 53x11 to get enough resistence at 55 rpm to do muscle tension.
As for your wattage assumptions, you're forgetting that in big field sprint you're getting a big draft effect in the lead out. And in the end when you punch it you're going to be putting out a lot more than 871 watts.
Sustained 871 watts would be super human. 871 watts for a sprint would leave a decent sized rider off the back of a Cat 5 sprint.