Sharing partitions with multiple Linux installations

[alex <radsky@ncia.net>]

I have multiple Linux installations, knoppix, Progeny, and two 
Debians, each sharing a common swap partition and a /home
partition. One of the Debians is used as a test bed for the
other.

So far, I haven't had any problem that I can detect by sharing 
/home and /swap but I also don't see any great advantage except
using less overall disk space.  I use GRUB for booting.

Would there be any particular benefit or problem to having
multiple systems share other partitions such as /var /usr or
others?