Personally, I think you should pay for games for what they're worth in terms of gameplay and re-playability with mods, levels etc. For example, I would gladly rent Return to Castle Wolfenstein just to play the single player to have my chance of checking it out and have no interest in the multi player which is crap over a slow modem anyhow.
I should have rented Max Payne instead of paying full price for it considering its a very short game and I haven't played it since except briefly with a couple of mods. The same with Black & White, just sitting on my shelf collecting dust.
However, I would always consider buying a game like Red Alert 2 Yuris Revenge because I'm collecting them and follow the C&C series but then its also much more than that. An RTS game has a very long replayability factor where each skirmish/network game is different. Then theres, Worms World Party which is an old game from last year that has so much replayability and is one of the only multiplayer games that works perfect over a normal modem connection.
well like i said, renting PC games is something you would never expect to see because technically anyone with a game copying guide and with a cd writer can pirate them.