I agree with you that real-time kernels are pretty much the mecca for gamers. It is absolutely critical that rendering threads get a tiny slices of CPU time at the exact right times rather than getting offered a big slice of CPU time at the wrong time and no CPU time when it needs it most. You can kind-of avoid the need for real-time kernels by just buying complete overkill hardware as most Windows gamers do, but I don't think this scales well into the future.. Linux desktops these days run a full SQL server, or even 2 as is the case with KDE. On the Windows side there are virus checkers on every PC. Sure, most gamers disable virus checkers as a no-brainer, but it really doesn't have to be this way with a real-time kernel that understands exactly which threads are absolutely time critical for user responsiveness and which threads can sleep for a solid second without the user even "feeling it".
You can boost/lower priority of apps all you want, but in a heavily loaded system running a regular kernel that doesn't actually guarantee anything in terms of responsiveness .. The only thing that can guarantees responsiveness, assuming you have sufficient hardware for the task, is a real-time kernel.