A much higher level of responsiveness from applications and operating systems. This is hard to quantify but if you've used a system like QNX as your desktop machine the continuous irritation at small 'freezes' of your machine would evaporate, you could control hardware directly from user processes without dedicated CPUs to drive time-critical stuff and so on.

Hard real time seems like a luxury until you've worked with it in practice and then you wonder why it isn't a basic requirement of any half decent operating system.