From a highly competitive (MLG) point of course you want a very focused and tight, arena style sandbox, and the BR definitely fits in there. However for regular MP gameplay, as I just said, a utility weapon that is too reliable leads to boring gameplay, and the sandbox not being used to it's potential.
It's not the case of 'highly competitive' and the rest, it's dependant on how you interpret the multiplayer experience. I play multiplayer to pit my own skills against a series of opponents, as do most of my friends, none of which fit into this 'highly competitive (MLG)' category you're talking about.
For this reason, the BR was supreme. It didn't break the sandbox in the same way the DMR does, for example, it wasn't devastating at long ranges; however it was reliable enough to properly facilitate this pitting of skill between opponents. In this respect I'm opposed to the loadout system, as for me and my friends who treat Halo as a game whereby challenge and competition is a draw (again distinct from the ultra competitive MLG style), having people on a level playing field is part of the point.
And that's why I said I enjoy a varied sandbox in singleplayer, because then it's not a test of human ability, it's about testing the limits and vulnerabilities of the excellent AI.