I'd love to leave my pet on the boss. But hey, I actually have a job to do, and that involves killing rages in a timely manner. It strikes me as highly irresponsible to leave a BM pet on the bosses just to squeeze out the DPS when pets are an integral part of...well, everything, in BM, whether it's AoE or burst damage.
Maybe you're talking about 25 man, but I don't have that luxury on H-10 man, especially when our rogue is soloing strengths, our spriest is soaking every courage spark, and our warlock is afflic and not very AoE bursty. It's basically me and our frost DK DPSing the Rages down, though now that spriest Mind Sear has been buffed, he's somewhat useful now (we do 2 waves at a time, in 10 man).
Edit: I mean, the enrage is a joke. Maybe you'd have a point if it was tight, but it's nowhere near a tight enrage.