The -rt (RT-Preempt) kernel patch is not in the kernel mainline because the Linux Kernel is focused on the implementation of a kernel for general use, and multi-tasking was a choice from Linus Torvalds when he began implementing it.
It is curious why the kernel is the way it is. The answer is that it is the way it is because Linus chose this way. And why he chose this way? Because he belived it was the better way, or the achievable way of implementing it.
Some people think that others design concepts is better than the actual one. But it is impossible to say that. Each one has its own opinion, and sometimes there is no way of benchmark comparison due to the complexity and possibilities.
There is the classical discussion between Linus Torvalds and Andrew Tanenbaum about monolithic versus microkernel. And there is many arguments to say that each one is a better model.
The same way, there is multi-tasking and real-time. Which one is better? Each one has its own advantages and drawbacks.
All in all, mainline kernel does not comes patched to be real time. Simple! This is just because of developer's choices and beliefs!
Best Answer
I don't think so. The patch seems to provide real-time scheduling which is very important for some enviroments (planes, nuclear reactors etc.) but overkill for regular desktop. The current kernels however seems to be enough "real-time" and "preemptive" for regular desktop users[1].
It may be useful if you work with high quality audio recording and playing in which even small amount of time may dramatically reduce the quality.
[1] Technically both are 0/1 features but I guess it is clear what I mean ;)