I'm not saying they don't help. As you correctly pointed out: if they didn't help, people wouldn't do them.
I'm not saying that increased bat speed isn't likely to help. It probably does help, so long as a guy can correct his timing to take advantage of the increased speed. Again, if it was more likely to hurt, they probably wouldn't do it.
What if a scientific study showed that working out 3x a week for a year with a weighted bat, increased bat speed by 15%? Should working out with weighted bats be outlawed? And so on, for the rest of my arguments. They just show that, and you seemed to agree, making the use of particular substances against the rules, seems arbitrary.
At the end of the day, I just don't really see the problem with at least some subset of these PEDs. They're not "cheating", if anyone can use them, if they choose. You still have to work your ass off, to be elite.
They should not be against the rules. If a player chooses to put his own personal health at risk, that should be their right. But frankly, I think a lot of the health risks of PEDs are overblown and stem from excessive use/abuse, as opposed to smart, controlled cycles of use.