Bungie struggles with one game at 800+ employees, seriously doubt they're doing three at once, Bungie is well know for being a mess when it comes to development of their games, Halo 2 and 3 are well known for having a shitty development, along with Destiny. Frankly Bungie helping out on Halo doesn't inspire me with much confidence either considering a lot of the veterans have left Bungie so I hope that's not true.
Buying Bungie would cost way more than all their purchases so far, It'd probably cost more than the majority of them combined and goes completely against what Microsoft has been focusing on which is AA-Bordering on AAA developers which they can build up, none of the purchases so far have even passed 200 employees, cost of Bungie alone could be used on 3-4 good quality studios that can be built up.
Exactly. Microsoft would be spending tons of money on a studio that clearly doesn't really want to be under anyone's command and in which many of the people who really made Bungie what it was during its Halo days are not there anymore. The money would be far better spent on acquiring a number of smaller studios and building them up over time. The best time to buy Bungie would be in the event that its back is to the wall, is forced to downsize, and then sell. 800+ is just too many mouths to feed for a studio that isn't what it used to be.