Sequels can lead to reiteration but they also give developers a chance to perfect their original ideas. When done right, sequels fix problems in the first game and give fans more of what they want.
Sometimes developers just need a second chance to perfect mechanics and polish game-play. Just Cause 2 seems to be on track to do just this. Sometimes the 1st game presents a great concept with execution that could use improvement. Without sequels we wouldn't have games such as Mass Effect 2 and Halo 2. Developers have the opportunity to evolve game-play and also in some cases an engine that's already been made.
Sometimes developers don't have the resources or the time to complete an epic story line in just one game. Trilogies would be impossible unless there were follow up games. Sequels give writers the chance to pursue more ambitious stories. Star Wars couldn't be one movie and Mass Effect wouldn't feel complete with just one game. Of course some games use this as an opportunity to ship a half finished game (Assassin's Creed). This shouldn't give follow ups a bad rep. Sure, all of the Matrix movies after the 1st one sucked but Terminator 2 was better than one and one could argue Aliens was better than Alien. After receiving constructive criticism, dev teams know what to improve on and have concepts to build on.
Unfortunately in this industry sequels are warranted by sales instead of whether game-play can be improved. In an ideal world, developers wouldn't have to rush games out the door and sequels to good games would be given without considering the sales. However, this is a business and developers do need to pay for these games.
