You can snap the current xbox app in windows 8.1 into a side panel, I don't know why you wouldn't be able to in windows 10.
Hmm... so my only Windows 8-based devices are my laptop (no touch) and my touch-based HTPC. Now, these are both single-display devices, so how does the screen splitting work with multiple displays? Also, how do you even do it with a mouse? On my touch screen, I swipe down from the bezel, and you perform the same action with a touch pad on the laptop. Also, what would happen to the second monitor?
I have no qualms with being integrated into Xbox Live so long as it's unobtrusive yet fairly accessible. I don't know if I'd want to go through the hassle of creating a split-screen setup every time I want to use Xbox Live and the normal PC stuff at the same time.
As for Plex, I'm not going to pay so I can stream my content from the PC when the xbox one can do it free of charge. I haven't actually looked at all the benefits of the Plex pass yet.
See... I think they would've been better served if they didn't introduce the newer clients through PlexPass. Now, it's not unheard of as Plex normally introduces their newer,
beta features to their paying members. These features end up rolling down to all users after a month or two. People that actively go searching for Plex are possibly aware of this extra cost. The problem with the PS4 is that Sony doesn't have a DLNA solution, which is getting people that don't understand Plex (as they normally wouldn't use it) to consider it.
Eventually, the PS4 and X1 clients should just have a one-time cost associated with them, which is just like it is for the iOS, Android, Windows 8 and Roku clients. Albeit, the Roku and Android clients are free for PlexPass users, and PlexPass users can also manually download the Windows 8 (Modern UI) binaries.
Ultimately, if you have an interest in sharing your media with various devices, Plex might actually be nice for you. PlexPass also includes a feature to save media to your devices, which includes any encoding that needs to occur for it to natively play. My friend uses that to put movies on his iPad for his son to watch.