Every next Linux OS is better than the previous one technogically and equipped with latest packages with more functionalities, latest kernel giving more hardware support, improved customization, more eye candy, may include technogically advanced latest file systems too..etc. Atleast thats what I have observed.....haven't read their brochures though!
If ur comfortable with Ubuntu 6.10 then stick with it, there's no need to upgrade. Likewise I'm comfortable with fedora 5 and haven't upgraded the distro/updated the packages since then. But yea I'll ofcors go for latest tech. when I buy a new PC, why even bother and waste my time in installing when I'm comfortable with my existing and perfectly working fine distro?
So u see thats the biggest advantage of linux. On old hardware u can put some older distro. But then older kernel doesn't mean weak kernel. Its open source remember that, that it has been tested by millions/billions across the globe with different hardware/platform. Older kernel basically means weak hardware support. So u can put old Linux OS, and can compile the latest software to give u the optimization, as they say the software compiled on ur platform works/runs the best.