By the same token, why should a pitcher be helped when a double into the gap is snared by the fielder and made into a single?
Or when, with a runner on second, a weak-armed OF can not throw the runner out at home on a single? Why should the pitcher be punished with a run?
Because, hopefully, it all evens out (somewhat) in the end, and pitchers who give up hard hits are more likely to give up extra base hits than those who give up weak flies. We play the game based on stats, and I think that is a decent stat. However, I would use TB/IP to make it more like WHIP. The problem with not using Innings is to look at these two pitching lines:
Pitcher A 5 Ip, 5 hits, incl. 2 2b, 1 hr, for a total of 10 bases.
Pitcher B 9 ip, 7 hits, incl. 1 2b, 1 hr, for a total of 11 bases.
You could also make a category of Toal Bases plus Hits. Me, I'd rather use something like opponents OPS, which takes these into account.