Believe me I have! But there are a lot of bits of software out there now that 'filter' out tracks (vocals, guitar, bass, drums etc) and these are just more interesting to play around with and - hopefully - to listen to.
The multitracks you sent me are especially exciting as I can use guitar solos to add a bit of flourish here and there, or throw a few backing vocals in. It really opens up a whole new world.
I've just done another track using your multitracks. Octopus's Garden, I Am The Walrus and What's Going On (apologies to everyone) - and as well as mixing the vocals to the backing, the multitracks let you mess around with the arrangement much more.
Have a listen and you'll see what I mean:
http://www.mashups.blogspot.com