Personally, I consider it an enormous shame that LBT is apparently best known for its large number of sequels. Regardless of the quality of the sequels, the original film was magnificent, and I wish it would receive more credit for being so. Alas, it seems that all the reviews out there that do praise the first LBT invariably denigrate the sequels to some extent as well, while the rest condemn the series as a whole, with no credit given to the masterpiece that started it all.
I wouldn't be surprised if a large number of the people who accuse LBT of "sequelitis" have never actually seen most of the sequels. Indeed, a large number of the reviewers on Animeboye’s link apparently stopped watching the series after a given number of sequels, and base their opinions of the later sequels on the words of others alone. (Some appear to be judging the films by the look of the DVD covers alone, which is irritating, considering that the cover art of just about every film past XI is cheaply done, and often poorly represents the movie.)
Perhaps LBT suffers from a stigma of sorts, due to being a show about dinosaurs whose intended audience is primarily children, and featuring large amounts of singing. (I despise the notion of drawing parallels between LBT and Barney,

but you see what I'm getting at.)
Admittedly, there
does seem to be a general consensus that the quality of the sequels has decreased over time, and there are some sequels (namely LBT XIII) and songs (“Oops-Eeps”, “Feel So Happy”, etc.) that are widely disliked by even the fans. Still, as pokeplayer984 pointed out, shouldn’t it be common knowledge that sequels are generally inferior to their originals?
Sequel quality aside, one thing that confuses me is why it is wrong for a movie series like LBT to go on for so long, but the same is not true of a TV series that lasts for the same length of time in terms of total minutes of footage. The combined running time of all of the LBT films (not counting the TV episodes) amounts to approximately 16 hours and 12 minutes; the rough equivalent of 21 to 24 episodes of a TV series in a one-hour timeslot. I know of TV series that go through that many episodes or more in a single season. So if a TV series can be considered successful for lasting for several seasons, what makes a film series containing many sequels a “zombie franchise” overdue to be put down?