The reason the "even good, odd bad" theory exists is because the even movies generally center on action, and the odd movies are more like actual Star Trek episodes (except for Enterprise and Voyager who believe more in "Screw peace, shoot them"), more quiet and peaceful and to most people boring.
*Trekkie alert* My analysis of the Star Trek films thus far...
Star Trek: The Motion Picture- This one needs no explaining, it actually WAS supposed to be the pilot episode of Star Trek: Phase II, but it stretched it out with their long special effects sequences and bored the life out of it and made it a movie.
Star Trek II: The Wrath of Khan- gave us a lot more action and the Starship combat that was missing in the first and it was quickly installed in most people's minds as the best movie of the series. It also gave us another look at Khan, probably one of the best Star Trek villains ever.
Star Trek III:The search for spock- Except for a few parts this film slowed us down a thousand fold from the last movie in order to bring back Spock. The only excitement was blowing up the Enterprise which wasn't really all that ground breaking since they get a brand new one that was identical except for the "A" on it at the end of the next movie. Again the movie was slow and wasn't aimed towards action, so it wasn't well recieved. Their attempt to make Christopher Lloyd a Klingon, though amusing, didn't live up to the awesome enemy we had in Khan in the last movie.
Star Trek IV: The Voyage Home- Well they thought of something other than action to liven things up, commedy. Yes it worked for this movie and it did very well, but in my opinion this movie is rather sickening after the first watch.
Star Trek V: The Final Frontier- I'm surprised this wasn't Star Trek's final frontier given how much this movie sucked. Being a guy that can't stand inconcistancies and I'm too stubborn to watch Enterprise anymore because of that, this movie is the bane of my existance as a Star Trek fan. Not to mention the story was completely lame itself. They attempted to add in some of the commedy from the last movie but in a more serious atmosphere it was completely out of place and awkward.
Star Trek VI: The Undiscovered Country- And with this film it seems they went back to what worked before, action and an enemy to rival Kirk, even if they didn't have a good inspirational quotes department for this movie and they had to pull out the Shakespeare. It was a pretty good movie for the original series to finally quit with, the alliance with the Klingons seen in the Next Generation was finally explained and Kirk's greatest foes finally subdued. Again, it seems that bringing Starship combat and action to the table makes a Trek movie better recieved.
Star Trek: Generations It seems like someone finally caught on to the whole "odd movies suck" theory (or just got sick of hearing about it) and wisely decided to drop the numbering all together, even though that didn't prevent fans from numbering the Next Generation films themselves. This film more than any other played out like an episode of Star Trek and I suppose fans hated it for that. I on the other hand really liked Generations, it bridged the gap between old and new and in effect the movie torch was handed down from the original series to the next generation.
Star Trek: First Contact- After Generations wasn't exactly universally loved they decided to throw out a balls to the wall action film which keeps to the "odd and even" trend that they ironically tried to avoid by not numbering the next generation films. I enjoyed it, especially the Battle of Sector 001 at the beginning which showed us quite a few new ships and of course the new Enterprise itself was rather impressive, having some of the Excelsior's lines only much more streamlined. The plot was a little out there, but it did flesh out Star Trek's back story a little bit. Still, I could've done without the time travel but then it would've just been another Borg story that we've already seen I guess. It was recieved better than Generations because of it's quicker pace.
Star Trek: Insurrection- Yes there's action in this movie, but it's few and far between and it's crappy. When you look back at it, this movie contributed nothing to the Star Trek universe. It wouldn't have even made a decent episode of the series which is what it could've and should've been instead.
Star Trek: Nemesis- Again with the route of balls to the wall action. The feeling that this is the last Next Generation film and possibly the last Star Trek film is apparent when you look at the nods of the head to the films The Wrath of Khan, and especially the Undiscovered Country, where this film ends in uneasy alliance with the Romulans like the original series ended with the Klingons. I admit the movie wasn't that great or original and had far out concepts like the Remens, but as a Star Trek fan I was waiting for ages to see a Starship brawl in the Next Generation films and I got the mother of all Starship brawls. (Admittedly it was much more exciting in the theater.) Reactions to Nemesis are uneasy though, a lot of trekkies love it, and a lot, not so much...
In conclusion, the "odd and even" theory is in the eye of the beholder. There's many different opinions, and there's many people who think all of the Star Trek movies suck. If you take the general concensus on Star Trek films then most of the odd movies were pretty subpar compared to the even ones, but it's just a coincidence, it's not a pattern that can't be broken.
Curses are fun, unless you take them too seriously.