The problem I find is that Android wasn't designed for the tablet. It was designed for the phone. The whole model OS was built with the assumption that it was going to run on very low spec hardware on a very small screen. Hence the limitations such as requiring all apps to run full screen, and the operating system's ability to kill an application (or the activity) at any time, leaving the developer to jump through hoops to make sure that information isn't lost. Tablets (and many phones) come with 2 GB or more of RAM, so there's very little need to manage applications in this way. Of course, they left out an easy way to actually close applications, so going back on that decision now seems difficult.
I think that the fact that Samsung has added multiple apps at the same time kind of shows that people really do want this feature, and that Google is being stubborn by not implementing it in standard Android. Even MS has realized that the simple 2 app split screen isn't enough for people, and will allow windowed apps on the desktop in Windows 10.
I think the major problem for MS right now is that Surface Pro is just really expensive. The starting price is quite high. I think next iteration, they should offer a model with an Atom/Baytrail processor (or whatever the current low power x86 option is) without a digitizer and include the keyboard by default, to bring the price down to what more people are willing to pay for a laptop. If they can get it around $400-$500, then I think a lot of people would opt for it over a more traditional laptop, and they wouldn't need to buy a tablet. Or if they bought a tablet it would be a cheap, 7 inch, $100 tablet that they could just throw in a bag and not have to worry about too much, but which really wouldn't be a productivity device. Once you have have 10+ inch tablet, it's really nice to be able to use it for real computing tasks, because it gets expensive enough that it's hard to justify such a high price for something that you're just using to read books, watch movies, and check facebook.