Positive - I fucking hate steroids in sports. I'm all for them for non-athletes, because I believe in freedom and I think they can improve quality of life greatly when used responsibly with medical supervision. In sport, it's only morally wrong because it's against the rules, but it should never be legal.
Negative - It would ruin the sport for a bunch of reasons.
First, it would be impossible to regulate. Preventing dangerous abuse would be as hard/impossible as preventing its use in general is now.
Second, not everyone will be willing to use steroids or be able to afford them, so guys who could have been great won't even compete. Allowing steroids would significantly decrease the talent pool. And allowing steroids in MMA if none of the other major sports allow it would just make this problem worse. Very few parents will let their kid train in the one steroid sport, especially if it's fighting. I think a lot of older athletes would feel the same way.
It's unnatural. Why is this bad? Watching athletes compete beyond the capacity of natural human ability is tantamount to watching fighting robots compete. You'd always know you weren't watching real human fighting, but an exaggerated version of it. I don't care about robot fights, or dog fights, or little children fights. I want to see the best human adult athletes compete. Exaggerating that for the sake of, what? Making MMA look like a fantasy comic book? I find the idea perverse.
Steroids would also taint the vastly underrated mental aspect of fighting. Steroids can greatly affect how you behave and your attitude. They can increase confidence, increase aggression, and through a variety of ways they can make you tougher. I love the psychology of fighting, and how mental states play into the game. By affecting these mental states, steroids take away one of the core pieces of true martial arts.
There's the risk of increased injury -- this is purely speculative, but still a legitimate concern.
Did I mention it's dumb to be the only legitimate sport that allows it? It would look so bad, it could kill MMA.