I use my arcade cabinet as a media server for the house (the HDD came cheap and is far too big for emulation alone!)
I keep all of my MP3's and video files in a shared folder on the machine. I've set it up so I can essentially access these files from all the computers in the house, and remotely via Orb/Winamp Remote.
I normally keep the cabinet on; Sounds enabled in MALA displaying videos, screensaver after 2 mins showing fullscreen videos muted.
This works great for having that 'attract mode' feel, while not being annoying if we're using the ps3 or TV in the room.
I only turn of the cabinet if it looks like I'm not going to be tinkering or playing on it for any length of time.
I do something similar: my cab is also a fileserver (well, it will be when I unmix the wifi in my house...) and is always running.
However, the switch on the top of my cab controls a powerbar that powers the marquee, monitor and audio amp only.
The computer stays ON (running Mala)
When I "power the cab" it powers the monitor/marquee/sound amp; to a quick "boot up to mala".
I rarely have issues with that. However, if I stay in a game, I sometimes have issues going out of it, and if I leave it in Mala, sometimes the kids will play with the controllers and start "something".
Having a "input enable toggle switch" could be useful, or something to that sense.