I don't know if I can give you reasons not to spank but I am not opposed to spanking (clarifying what 'spanking' means to me - a swat on the tush with your hand) but we choose not to spank.
I say that and yet, our two year old, when she gets upset in the car (which really is quite frequently) will take off her seatbelt. Our solution is to stop the car, swat her hand and put the seatbelt back on. It's really slowed down the behavior and til we added the swat, nothing seemed to work...so maybe I am in the 'spanking' category.
Anyhow, that is the only time I swat and that's only because she has me at a disadvantage. We're on the road and I don't have a good place to give her a time out. Believe me, I even tried pulling over for a time out but she seemed to think that was funny. At home, time outs work because she is removed from the rest of the family and she really hates that. Afterwards we discuss why she was in time out and that just seems to be a good solution.
My son is almost seven and I've only spanked him once and everybody always compliments us on how sweet and well behaved he is. Of course he has his moments but he's not being willfully disobedient...often, he just has too much energy and drives me nuts
Time outs still work wonders on him and, I'd say, on average, he gets one, maybe two time outs a week, if that.
Okay, just my ramblings. I just believe that unless you're abusing your child, how you discipline is your business and everybody else should keep their opinions to themselves.