I understand and respect that consoles heavily outweigh myself and other PC gamers when it comes to buying power, but (and this might be a little selfish), I can think of a good reason why porting from console to PC is a bad idea: It won't take advantage of what the PC has to offer.
While there have been a couple exceptions (Chronicles of Riddick being a good example), most of the console -> PC ports I've seen have a noticably different feel than a straight PC-developed title. For example, Mass Effect 2 has the spacebar do about five different functions, rather than allowing us to map all those functions to separate keys. Nothing gamebreaking, but frustrating nonetheless. What -is- aggravating is how low resolution the textures can be in the game, especially on characters. It looks fine on an HDTV, but it looks cringe-worthy on a PC.
If you develop a game for the console with plans to port it to PC later, what business incentive is there to optimize the port with better visuals and customizable controls to take advantage of modern PC's? I can't think of any, as most of your sales will be on console anyway. And that's my problem. If you're going to do multiplatform development, tailor the game to each platform and utilize each platform's strengths.