I don't think it's fair to keep cats inside personally unless for a very good reason, e.g. cat is disabled, has feline HIV, has been rescued and never known any difference etc... It just seems cruel to me. After the first month of having Eric (while he was adjusting to his new home) he was chomping at the bit for 'more' so I was (and he was) very glad to finally let him outdoors for the first time. He still stays in at night and if the weather's naff then he's hard to prize off the top of the wardrobe but otherwise he's out a fair bit. I can see the difference it made to him and how much he loves it and it would be wrong of me to deprive him of that just because I worry about his safety (which I do).
I think the difference between a cat who prefers to be inside (I've had one of those) and a house cat is choice, I've no problem with cat who wants to laze around inside all day but I don't necessarily think it's fair to keep cats indoors for our own convenience.