One other factor when talking about indexed vs. non-indexed shifting compatibility is that the newer 9sp and 10sp cassettes (and maybe older ones too) have ramps built into them to aid in the shifting. The chain rides up and down on the ramp to provide smoother shifts. If you have the full integrated system, where the shifter clicks in and the derailleur moves a certain amount corresponding to the cog spacing, there is the ramp that makes it happen so quickly and relatively quietly.
So now, take away the indexed shifter, and you still have the smooth changing of the cog. If you have very narrow 10speed, the shift happens with a light touch on a friction shifter. It really works very well and is very quiet, you don't even have the click of the indexed shifter anymore. The only issue with a 10speed setup like this is that most older derailleurs won't swing all 10 cogs. I can use 9 on my setup, but that's really enough anyway. The other benefit of this system is that the wheels themselves are better nowadays.
I've had my older bike set up like this for a few years now, and I just set up another BF member a few weeks ago like this that had a different derailleur and shifters than mine and it worked just as well.