Now, for those of you who don't remember back when poison counters were a fair and likable mechanic, I'll give you an example.
This is Marsh Viper, one of the original cards with poison counters. Why is it so much more balanced than a creature with Infect? First, look at it's cost. It's a 1/2 for 4. For what it does, that's actually a fair price. Second, and most importantly, it gives a set number of poison counters, unlike Infect, which varies on the power of the creature. But, rather than pull out card comparisons, I think it would be best if I just ran down some arguments that I've come up against and explain my position on them.
#1 - "Infect is balanced because there are cards to deal with poison counters"
Actually, there's only 1 card in all of Magic that removes poison counters: Leeches from way back in Homelands. Now, if you're talking about cards that prevent you from getting poison counters, that's not necessarily true. You're probably thinking of Melira, Sylvok Outcast as a solution to Infect, and it is; if you're running green! Where's the white, blue, black, or red solution for Infect? You can't just claim "there's a solution" when only 1 colour in block has a viable method of prevention and only 1 card in the entire game exists that can solve the problem.
#2 - "Infect creatures are really expensive"
For the most part, yes this is true. A lot of creature with Infect are overcosted in a vain attempt to make Infect balanced. However, Skithiryx, the Blight Dragon is a reasonably priced 4/4 Flying Infect with the possibility of Haste and regeneration for 5. Hand of the Praetors gives players poison counters and powers up Infect creatures for 4. Blighted Agent is unblockable for 2!
#3 - "There aren't that many good creatures with Infect"
You mean besides the ones I just mentioned? Well, even if that were true, there are a multitude of ways you can give other creatures Infect. Grafted Exoskeleton gives a creature +2/+2 and Infect for 6 (or 4 if you have the right cards out) and Glistening Oil and Phyresis give creatures Infect as well. Tainted Strike is an instant-speed powerup and it gives Infect. Finally, Triumph of the Hordes could be seen as a better Overrun since it's cheaper and much more deadly.
#4 - "It's just as unfair as Wither and you never complained about that"
I don't even think this warrants a response. Wither was a much better mechanic because it forced you to chose whether or not it was better to kill an opponent's creature in exchange for weakening one of your own or letting it deal you damage so that you could swing with a better creature. One of my friends has an excellent Wither deck that I find is one of the most tactical decks I've played against recently for that reason. Infect is not a choice. You have to block a creature with Infect whether you want to or not because you will die and there is no way to recover from a hit with Infect.
Those are the biggest arguments I've come across in favour of Infect. If you have anything you'd like to say about Infect and attempt to convince me why it's balanced, post them in the comments below. I am willing to take back all of this if someone can prove to me that Infect is a balanced mechanic, but I find that very unlikely.