Yeah, I noticed that when I rewatched the last few minutes of Doomsday a few weeks ago. IF NOT for that cliffhanger, there wouldn't have been as many fans complaining for the past 15 years about it ending (though, on a related note, I like that the current comics are basically going the angle that season 3 would've started off with).
Legion of Superheroes had a similar problem, where nearly everything was pretty much wrapped up, there was the whole "the future looks bright" thing, and then...BAM, the original Braniac is shown floating in space, giving a speech about how "evil doesn't die...it EVOLVES". However, the problem I have with THIS one is that, with Sonic, the show was (to my understanding) almost renewed...this, on the other hand, they KNEW that Kids WB would be no more. They KNEW there was an incredibly slim chance of renewal, especially since they would need to go to Cartoon Network before any chance of that happening (and it never did go over there).
Really, I think that any series that has something like that, a finale that includes a short cliffhanger, should just remove the cliffhanger as a "deleted scene" unless they're ENTIRELY certain that they're getting renewed. Otherwise, it just pisses off the viewers.
It all depends on the series, really.
In a comedy series with standalone episodes, it wouldn't make sense to do a "grand finale". With an animated sitcom, the same rule applys, but it all depends on the strength of the continuity (like King of the Hill having closure because it has continuing stories).
However, with action shows, it feels like an insult to the viewer if there isn't SOME form of closure. I mean, they don't necessarily have to wrap up everything (some people enjoy mysteries left unsolved), but it'd be nice to see something.