I prefer my stories to have some finality to them (in other words, I prefer them to have an ending), so the idea of sequels continuing the story isn't what I'd like to see. I will make an exception for planned trilogies (such as Mass Effect), because there is an ending planned and the proposition of making all that into a single game is a bit unrealistic. Anyhow, the key point is that numerous sequels are unneeded and obvious cash-cows. When a story wraps itself up nicely, it should be left that way.
As for sequels following an unrelated story, but still taking place within the same mythos, that's more of a spin-off. These can be viewed as just as unnecessary as cash-cow sequels, but it (usually) respects the fact the previous storyline ended. In many cases, this is preferrable... but still not exactly the desired course of action. What's done is done, leave it. And no loopholes!
Then there's the act of fabricating an entirely new setting and story (effectively a stand-alone title), but having the game remain under the same name. These games are what make up most of the great franchises of gaming, most notably Final Fantasy (which has regrettably fallen from grace). When each title is a stand-alone, but still goes to great lengths to create massive world and storyline to follow... well, there can't be really any complaints about that practice. The name of the franchise does draw a lot of attention by itself, but those franchises are still held to a high standard (which is why the FF series gets a lot of flak despite the game being quite good in their own right; the fans want works comparable to their older masterpieces, not merely "good" games).
Meh, it's a tough call in many cases... but I prefer it when developpers don't make it obvious that they're making the sequel to cash-in on a previous title's success. If a story is done, leave it be.