Well I only switched last week because it's a fair amount of hassle switching between ROMs and having to reinstall apps, etc. I know you can use Titanium and other tools, but it doesn't always go smoothly, and to get the best experience it's often worth taking more time and doing things manually.
Also, although it's unlikely, I'm always nervous about flashing new ROMs in case they screw up my hardware. On my old G1 I waited about a year before attempting to root it just in case I was left for the rest of the contract with a brick. This time I've gone for it a lot sooner, so there's more at stake.
Finally, it works - and it works well. With root, ext functionality, etc. included, there may be little incentive to get a newer version.