Opera didn't have much choice after their 2012 earnings were well below estimate and the market for there desktop browser, despite how awesome it is, is just too small. Their only strong point is mobile. I don't know why they haven't open-sourced the old desktop core yet, I suppose it's possible that they are holding on to hope that they can mold most of it around webkit and keep it as good as it's always been.
However, I wouldn't dismiss Microsoft so quickly. They're a serious problem on many fronts and will probably switch sides once this battle is done, but they have plenty of reasons to oppose a WebKit monoculture, at least. When you don't have many allies, you take what you can get.