Well it's either they do that or they just rush and make a new game every year like sports games. I'd much rather them do very polished main releases every few years and have optional filler stuff in between to be appetizers for the diehard fans.
Not sure about most people, but I can wait an extra year between releases on a series I greatly enjoy.
For example:
Halo 3: ODST didn't d much for the overall narration. One can skip ODST and not really miss anything from the overall Halo universe.
Halo Wars was a fun diversion but most of the Halo fans I know didn't even bother because "not a shooter, bro." I enjoyed it, found the controls to be excellent for a console and then MSFT shuts down Ensemble. Ha, guess that's the end of an RTS Halo.
Halo Spartan Assault - let's just not talk about this.
Halo 4 - Why is this even a Halo game? Why did I have to re-familiarize myself with characters and hunt down a book which then got thrown out the window? But this is in the MC collection? (If you haven't noticed, I really despise Halo 4. haha.)
Beside my Halo 4 mini-rant, those games in between major Halo titles are almost throw aways in the general scope of the series. Now instead of enjoying a joyful romp through something like say Halo Wars 2 I get to replay Halo 2 with the shinies (now that I mind this much, but my excitement won't be as measurable come Halo 3/ODST/4 Anni versions. After Halo 2 Anni I want nothing but Halo 5 (just to see if they can salvage this mess) or Reach Anni.