A consolidation of power, could be one interpretation.
That's the argument I've heard against it. "Better the devil you know" and "ATF is smaller and easier to keep an eye on".
But ATF also has only one real purpose, and that's guns. Both ATF and the FBI are under the DOJ, so keeping them separate just ensures there is an entire hierarchy of career Feds who are concentrating solely on guns and don't care about scandals because there is no downside to scandals in the ATF. Moving it to the FBI means that it becomes a small part of what the FBI does and to a large degree can be decentralized rather than having an entire agency trying to figure out how to ban ball ammo that's been in use for decades or deciding that a shoestring is a machinegun.
