Well, to be completely honest, the books were written to make a cannonical backstory. So if you even remotely like any part of the Halo Story-line, then you cannot just conviently forget the parts that you dislike. The Battle Rifle (Who's apperance in Contact Harvest gets me angry due to it still being in design during CE), makes it's first, true apperance during Halo: First Strike/Halo 2, which, are both as cannonical as the other in the story.
It's not like the games have more value than the books, in term of story. So before we start trashing the books, let us keep that in mind.
As for the Battle-Rifle appearing in Halo: Reach, I believe that it should be possible, and available (as far as story goes). For Gameplay? I'm not exactly sure. In Halo 3, I've learned to master the Battle Rifle, and use it with extreme efficency. However, when playing Generals and Brigadiers, it's not even fair. So, if you want a fair, low learning curve game for average players, remove the BR, trash it, get rid of it.
But, if you're looking to keep within the story, and keep your hardcore players content, keep it. Let us run rampant with the four-shots, or the one frag head-shot. That's my opinion on the matter. :)