Its microsofts program which they release for free. You should stop moaning and be grateful or use something else.
Yeah, we do them a favor by using it... they develop it to be used and surely the experience of the user is a key part of development.
I do not think anyone here can comment on Microsoft's actual reasoning behind it but I am pretty confident 'enriching' the 'Windows Experience' by adding IM capability to keep people on-side and being able to distribute advertisement is part of the whole goal.
You would think they would continue to keep people on-side by letting people use the app in any version that is actually capable of doing so. 8.5 is, there is no real reason to stop it working other than "We just can".