http://www.hardballtimes.com/main/article/base-stealer-intangibles-part-1/
http://www.hardballtimes.com/main/article/base-stealer-intangibles-part-2/
Apparently, it's a grand total of 0.8 runs per season.
Awe-inspiring.
EDIT* Can't say it with that much certanity, but it's very likely that the effect is that small.