I don't see much point in having multiple distributions on one computer, as long as only one of them can be run at a time. But then again, I'm not one of those who have too much spare time to experiment with 100+ Linuxes and reboot every other minute
I imagine that if I wanted to install several distributions, I would create
- 1x /boot
- 1x /home
- Nx / where N is the number of distributions I install
Then I'd use /home as every distribution's home directory (would not format during install), and /boot too (same thing, except that I would only install the first distribution's bootloader and skip that with the others). That shouldn't be problematic. The problem, however, would be like you said -- kernels overwriting each other. This could be solved various ways, one way being the manual (or scripted) renaming of the kernels; one can easily manually rename the kernel files in /boot and then just edit bootloader's config to point to correct files. Actually it became even more easier to use symlinks, so during upgrade one would only need to rename the kernel in one place. It shouldn't be that bad, because (at least on binary distributions) kernels are not upgraded every other day (not on those I use, anyway) for nothing, and when they are, it doesn't take long to do some renaming. If you compile the kernel yourself, you can do it during the process.
I haven't thinked of this too much, but it might be possible to only have one kernel for all the distributions too; that way having the newest kernel would be easy (though you might have to drop off automatical binary updating of the kernel, if the distributions used different methods/programs for that); in the bootloader config you would then just point to the correct root partitions. No idea if that actually works, but then again, no idea why it would not. If you like to use common partitions for /home and /boot to save space and effort, why not do the same with kernel -- why have three or four separate kernels of the same version when you can only have one that works for sure?