Imagine missing a gear with all those horses - A mate of mine was racing his renault5 turbo (which had been tuned to the max) when he mis-shifted accelerating out of a bend it wasn't a pretty sight as his engine blew.
That's probably not the reason he bought an auto but at least he won't miss a gear.
Also I was reading an article about a couple of american lads who had super tuned (M5 beaters) i6 e39's. One was an auto and his reasoning was that he'd bought the car BEFORE he even entertained an idea about tuning it (although he now wished he had gone 6 speed - hindsight is a great thing).