Firstly, what's the point of buying an over bloated PC and then going through the hassle of sorting it out so it (hopefully) runs your DAW without incident? You can of course call it lazy but most people have more exciting things to do.
So many hardware incompatibilities with recording audio etc I'm not sure what you mean.
Off the shelf machines generally come with at best, laughable support from people that know nothing apart from how to read the script they have in front of them, at least here in the UK. Why waste your money on an expensive warranty?
Of course you would be an idiot to take your PC back if sonar had a bug, who would do that?
It's actually cheaper to have a custom build than it is to build one yourself if you are any good at making deals and it's guaranteed to work straight out of the box for making music with the software of your choice. That's what I call a guarantee.
I don't see why a rational human being would not invest in a custom built daw by a reputable company if they wanted the best chance of a hassle free experience, especially give that a musician may not have much PC knowledge. Also i would say it depends on the type of music you are composing.