There's no need to let the fabs sit idle. Elpida Hiroshama plant can be converted over to make NAND flash chips. Cutting Dram supply which would help stabilize the Dram market. Of course the NAND market would probably be in over supply, but that could lead to a boom in SSD's as prices move lower. Micron also has an R&D fab in Japan(formerly Toshiba). Rexchip can continue making Dram mobile chips keeping Apples business. Micron has been battling these government sponsored Dram semiconductor company for years. Good old fashion American innovation has kept Micron alive. Hynix should of been bankrupt or bought out by Micron by now if it weren't for the unfair government subsidys keeping it going.
Micron has a lot of experience converting purchased fabs to their technology: TI, Toshiba, Inotera. All of these conversions seemed to take a lot of time and effort and expense. These fire sale fab purchases always seems like an incredible deal at the time Micron makes the purchase, and a not-so-good deal a few years later. Micron owned the former KTI fab in Nishiwaki which is now part of Jazz. So they must still have some fab experts in Japan that can help integrate the Elpida fabs.
I think the only winners here will be the Elpida workers still standing. Micron is not even going to use all of the wafer capacity it's paying for, and I don't think whatever modest technology advancement Micron gets from the deal will have proved to be long lived enough to justify the expense after the time & cost associated with assimilating and converting to Elpida technology. This is a break even deal at best for them, and it certainly won't be enough to let the good times roll again in DRAM.
Some of the reasoning appears self-contradictory, like gaining production capacity only to idle it. Micron and Elpida have different technology, so the capacity won't pick up for a while. There is also no government support for a micron-elpida entity through hard times, which micron could better weather without the burden of Elpida. Didn't Micron consolidate its own fabs recently? It is trying to lighten rather than bulk up.