Ok... so I guess I "get" why people care... I just don't "understand" it. If that makes any sense. Or perhaps "agree with" would be a better choice of words.
Allowing for a poor sequel/prequel to tarnish your opinion of other movie(s) in the series seems completely illogical and entirely based on an emotional response, something which simply doesn't happen to me when it comes to movie series, but which I suppose isn't something anyone can help.
Anyway, thanks to those of you who explained it to me.
KM