The highest I would go is 96 KHz, and that’s only if I had microphones, pre-amps, monitors, etc. that could actually take advantage of such a high sample rate. How else would you be able to capture and hear it otherwise? And even then, most playback systems don’t go that high. Most people don’t even hear past ~17 KHz! So you’re really catering to dolphins, dogs or other animals that can actually hear those higher frequencies. I don’t know. Why do you think 44.1 KHz is still the standard for CD reproduction? You would think that by 2012 we would have moved past something so ancient, right? 
And let’s not even get into compressed formats, like MP3 and AAC, which is the quality at which most people will probably end up hearing your music.
I could go on, but the main point is that 192 KHz is simply a waste of resources.
Yes, that’s basically what it boils down to. The ASIO meter measures the time it takes to complete a task in Cubase. The more time it takes, the higher the chances of pops and clicks because the CPU is not able to keep up with the demand. If that’s the case, then you need to raise the latency. If it’s already at the max it can go, then you need to resort to other methods (i.e. freezing, bouncing, etc) in order to free up some resources.
I agree, try enabling Hyper-Threading and see where that takes you, if you must insist on 192 KHz. To each his own.