This really depends on the game. If you have a game in which you can be good or evil, for example, then you really need multiple endings in order to flesh this out fully to its logical conclusion. Just one ending is good, but two would be far better.
KQ6, of course, had a good system: a sort of built-in "easy" or "hard" ending. I thought this was a really good idea, too, since not only the endings but a huge part of the game was played from 2 different angles, adding to replayability. (ie. you don't get to go to the land of the dead if you pick the easy path.)
However, I voted for one ending, since I think that, overall, multiple endings are more likely to be annoying, since you would not have as much closure as in just one ending: in KQ6 this is less so, because the endings are rather similar, but in Metal Gear Solid, for example, the two endings decide the fate of one particular character, so ultimately whether or not this person survives is meaningless, because it's reduced to "which one would you rather pick?"
I think a really good method of multiple endings, though, is if you have an intricate game with really intriguing philosophical and story-related points which are only revealed by playing both endings. For example, if you had a game in which you are unsure whether or not the main character was a clone, or a robot (think Blade Runner) you could have it so that only be comparing and analysing data from both endings would you discover whether or not s/he is a clone/robot. (Yes, I absolutely adore Do Androids Dream of Electric Sheep.)