First, let's get it out of the way right off the bat. He used steroids. He has admitted it, and I don't mean tacitly by "not talking about the past."
Now for the stuff that matters.
Mark McGwire is the career leader in home run frequency. Once every 10.6 at bats Mark McGwire hit one over the fence. That is better than Babe Ruth, WIllie Mays, Barry Bonds (who also should be in the hall when his time comes), Hank Aaron, and anyone else of any consequence that ever played baseball at the major league level.Perhaps Josh Gibson had a better rate, but he didn't play in the major leagues (which is admittedly a bigger injustice than Mac being snubbed).
So people will say that he only hit home runs because he took steroids, I disagree but also say so what if he did. He played in an era tainted by steroids and it is unfair to assume he was a) not the best home run hitter of his generation despite the numbers that indicate otherwise and b) unfair to blame him for an entire leagues worth of folks who aided their weight lifting recovery by increasing their testosterone levels.
We now know that Ken Caminiti was correct when he said, at least half of the players in his generation were taking roids. Though it was unlawful to take certain types of steroids (Andro was not one of those), it was not against the rules of baseball. And if we can all agree that Caminiti was right about the numbers (or that Canseco's guess of 85% was right) and that steroids increase a players production then it stands to reason that the best of players from the 90's were all cheaters. That also means that if you believe steroids disqualifies one from being in the hall of fame than only the worst 15-50% of players from the 90's are eligible. Who wants to see Mike Gallego in the hall? Walt Weiss? Jose Lind anyone?
This is clearly presumptuous. The truth is, we don't know who used illegal substances outside of Jose Canseco, Ken Caminiti and Rafael Palmiero. So what we have to do is use the facts as they are to determine who performed best on the field. Mark McGwire is clearly one of those guys if you consider just his numbers.
The moral indignation is comical to me, though I admit I felt that way once. If anyone would like to argue morals I would like to point to the the fact that the man who made sure that Josh Gibson died having hit all 962 of his home runs outside of the Major Leagues is a proud member of the hall.
The hall also houses a murderer in Ty Cobb, an admitted cheater in Gaylord Perry and several Giants players who benefitted from scoreboard operators who let them know what pitch was coming.
I will admit, after the congressional hearings I was not sure about Mac. Now that more facts have surfaced it is clear that McGwire is not a member of some immoral minority in his use of steroids (which in his case we know to be a legal over the counter supplement at the time). So the question becomes do you stop an entire generation of players from ever being able to enter the hall? For me the answer is clearly and emphatically no. Because of this Mark McGwire should be in the hall.