1. Cheaters are going to cheat, and you can't catch everyone, so just allow cheating. ("So what the current system does is enable that, because it places constraints that are impossible to enforce.")
This is a bullshit argument. If something's not completely enforceable (which nothing is), then just forget it. Some people don't pay taxes: dissolve the IRS. Some people commit murder: murder should be legal. You have to draw the line somewhere. People will push or cross the line, you will catch some, and life goes on.
2. Doping in sports should be a test case for human enhancement.
This is a more reasonable argument. I'm not sure it's right, but reasonable people can discuss whether the intent of sport is "measuring personal accomplishment" and "test-bed for medical intervention".
Note that a parallel argument is happening in Formula One: should teams be given more leeway to build cars so innovation occurs as a result of competition.