There's definitely a whole lot of good reasons why doping should remain illegal. Here's some potential one's off the top of my head:
1. Unsustainability. If you don't draw the line somewhere, the win-at-all-costs type of person will do whatever it takes to beat his competitor, which in a doping world necessarily means doping more than your competitor. The natural body has built in safety mechanisms to prevent catastrophic failure (eg you'll pass out before you can run yourself to death), but there's no such safety once drugs get involved and the body is artificially pushed to do beyond what it is adapted for. Athletics would devolve into a contest of how many drugs one can do before irreparable injury or death.
2. Legal doping would raise the question of when it is ok to start. College? High school? Middle school? PED's acting on your endocrine system are going to have serious developmental consequences that will only be more severe the younger you start. But if everyone else is doing it you won't have a choice if you want to run with the best, right?
3. Competition will become inaccessible to all but the most wealthy and privileged. It will be impossible to participate independently from the Nike's or Putin-led governments of the world.
4. This point is more philosophical, but IMO the most important. Sport will ultimately no longer be a contest to see who is the best athlete (ie the right combination of talent, dedication, passion, willpower) it will be a competition to see who is the best doper. Which means whoever is the wealthiest with the best technology wins instead of who works the hardest. We already live in a pharmaceutical culture that demands instant gratification and easy fixes, but sport is one of the last remnants of a way of life that rewards practice and patience. Opening the doors to doping would destroy that in favor of the "fix it with a pill" mentality. To me what makes running thrilling is that it's sort of a poetic reflection of raw struggle in the human experience. Once doping is the thing fueling that performance what have you got left? Something that IMO is not much more interesting than watching robots scoot around a track.
If you want entertainment go watch WWE or an action movie or something. As for me, I want to experience something real and something ultimately human.