Back in 2011, when Nokia decided to go All In Windows Phone, I decided that this is the best thing possible. Remember, Google was purchasing Motorola Mobility, and companies where coming out praising Google in public, but worried behind doors. Google has done well, making Motorola it's own company really and Google making phones with other companies, like the Nexus 4. That ended up well, for Google, however this is entirely different.
Microsoft is becoming a OEM. They didn't buy a small company. I mean, if they bought Kyocera, it would be different. They are small, and not a huge player. Nokia is one of the pioneers of the market. A beast, like Motorola was. HTC, Samsung and even LG aren't praising Microsoft like everyone did when Google bought Motorola. They now have to wonder, do they compete with Microsoft on their own turf ... try to dethrone the person MAKING Windows Phone ... when all of these companies are ALL READY on the fence on which platform to support? I think we will start to see less and less support from these companies, and Microsoft will have to mold it's own future.
However .. Nokia and Microsoft all ready had this. Now, Nokia and Microsoft being together, they will be able to build better products. Nokia can help Microsoft to deliver some basic features customers want -- help build a better Xbox Music App -- and help get more developers to make apps for Windows Phone.
In the end, I think this is a great move for Microsoft. It's not good if they wanted to gain the trust of other OEM's. However, I think they understand developing your own phone is a better way to be successful.