Renting is the best way to find out if a game is actually worth buying or not. If I can beat a $9.00 rental in three days, it just makes no finacial sense why I would've bothered buying it new for $60+ based on a review, or the preception that I might like it.
Renting games gives you more opertunity to play all the games you want, and try out some of the other 'if-y' titles with very little risk. If you end up liking a rental, and it hasn't been betten at the end of the period, purchasing it would be a good call. Or renting it again and finishing it for $42 cheaper than buying it.
Renting games can get you into a series, such as the Devil May Cry series, or it can illuminate what ganre you may actually be into, like FPS or RTS or even NFS Undergound kind of stuff.
In my opinion, I never buy a game unless I know for a fact I'm going to love it and replay it more than twice. Perhaps by renting the game and then buying it actually causes me to lose money in the long run, however, the ratio of games bought after renting to games never played is quite low, which means I've been doing some risk-free research into finding a game that I can truely enjoy instead of relying on fan-boy testimonials or biased game reviews. It also eliminates the need to actually try and like the game based on the fact that I blew $60+ on a title. There is no need to justify liking a rental, unless it actually is enjoyable. If you like the rental, more often than not, its worth buying and you'll be happier with the replay value.