In my case, I hate movies that didn't follow the source material when and because the script produced is so much worse than the source, the script essentially takes one element from the source and makes a totally different story about it, or, in some particularly egregious cases, actually reverses the theme of the source. Many of them encompass more than one of these reasons. You're right in that in the second case I should just ignore the source and deal with the movie on its own, but the producers have explicitly linked them. It's too bad that the WGA requires certain credits and that studios tend to parlay them.

However, all that said, while I hated (Hated, HATED) Minority Report (for all three of the above reasons), I thought that Spider-Man was passable. I've not seen SM2 yet, but folks tell me that there's even more character development, which is good, since that's mostly what Spider-Man is about.

(On a side note, it's interesting that Kirsten Dunst has suggested that killing off Spider-Man might be a good premise for a movie since the character that her character took the place of in the first movie dies when falling from the bridge. In a rather horrific manner.)