The Continuity of Star Trek IS and historically HAS BEEN a key element of Star Trek. 40+ years of stories and history that collectively creates a fairly consistent, large universe, to the point that computer software, and several books, are dedicated to illustrating and exploring this universe as a single entity, and sites such as Memory-Alpha attempt to organize and chronicle the same.
Get back to me when you can reconcile the events of Star Trek: First Contact and the novel Federation or Where No Man Has Gone Before and the novel Enterprise: The First Adventure or Balance of Terror and Minefield. Or the fact that every piece of Starfleet tech underwent a radical change in just a 18 month period between Star Trek: The Original Series and Star Trek: The Motion Picture.
The thing your missing, is that fans will always explain away discontinuities if the story is worth it. You could have simply placed the film on an updated, but not radically different, Enterprise... replace the original actors and given us a Nero who had an axe to grind and somehow stumbled upon superior alien technology. Then like all the other major turning points in Trek... allow the fans to hash out how it all fits together. And allow the general audience to enjoy a story not weighed down by writers trying to show how clever they are. And a story not weighed down by the sheer stupidity of sending a 160 year old ambassador on a solo military mission into enemy territory with enough dynamite to rearrange the universe and the whole 'cadet to captain' nonsense. YMMV.
Novels are not considered Canon, certainly not by Paramount.
Your original point was that continuity was somehow important to fans to enjoy Star Trek... I'm just showing that there is plenty of Trek out there that does not respect continuity that does quite well.