There are always exceptions. My Panasonic stereo system sure as hell doesn't sound any more powerful or punchy when compared to my Spirit Absolute 2's. I never listen to my stereo system anymore, in fact, I've put it in a closet to make better use of the space it consumed.
Im sure ill get flamed for this, but I still use my stereo system, with a pair of Wharfedale speakers. The response isnt all that flat, but by positioning them well in my studio, and placing my 'listening / mixing' stool in the sweet spot, I've found that whatever Mix I come up with sounds good wherever I play it.
If the speakers you use for monitoring are good, use them for whatever. Just dont use them whacked up when you have a party in your house, cus it'd be a shame to blow them while your pissed as a fart.
I use mine for everything from listening to mp3s, to video games, to lugging them around in my car for back seat speakers! I tell ya, monitors are loud buggers.
I still need to figure out a way to hook up a bunch of diffent systems to my computer and switch through them quickly for mixing. I'm not home often enough to use my monitors to listen to music through. I wish there was a way to hook my computer up to my car stereo too. Hmm... My car has a standard power outlet... Hmm...
Actually, it's not some clever name I sat up all night thinkin up. It's a song title I used on the fly trying to get back into the site when my other profile had technical difficulties. Little did I know I would wind up stuck with this title.