Eh, you lost me when you compared a 2 and a half hour movie to 96 hours of serialized television. Also, your example is a solid answer as to why a Dexter fan wouldn't give up on the show and stop watching with only a few episodes to go. That much should be obvious, but you failed to mention the bit about reading spoilers of the ending weeks before the show ends.
Try this. Look at it this way: You invest roughly 90 hours of your life, spread out over the course of several years, on a television show. Why give in to read spoilers of the final episodes of that TV show when there are only a few hours to go? Why spoil yourself by reading the ending of those 96 hours of television that you'll have invested...rather than simply watching for yourself?
Make sense? That was my original point, anyway.