The origin helps establish the more modern, complex characters that viewers demand and sets the mood and stage for what's yet to come.
Do you have an example of the more complex characters viewers demand? I'm just curious because I don't get the same feeling you do. Or maybe more of an explanation of what you mean?
I regard the first episodes of TNG, DS9, VOY, and ENT adequate origin stories. They all establish a proper BEGINNING of an era.
By more complex characters, I'm referring to the way characters are allowed to evolve throughout the series; having the ability to apply the skills or knowledge in one episode based on the character's experience from previous episodes, or previously established history.
TNG used this to a certain extent, and it was used more with DS9, VOY, and ENT. For the most part, the TOS characters were extremely static. Typical to story-telling style of the time, viewers were supposed to accept that those characters had always been middle-aged officers on a starship, and would always remain that way after the show is gone.
Don't get me wrong, I love TOS and all the characters for what they are. But as my personal tastes have developed, I find that I prefer TV shows that I can treat like super-long movies: I want a defined beginning and end; I want intelligent, creative writing; I want complex, intersecting plots; and I want characters that can adapt, react, and realistically change based on world they're exposed to week-in and week-out.