I don't know why you would need two servers on one line. Isn't one enough?
For myself... There are a few web apps that either can't be installed on SME or I have not been able to figure out how to. Much easier to setup another CentOS box to host things like Scalix (not that I have that working yet...), Zoho Virtual Office (until I get Scalix running), and AdventNet ServiceDesk Plus.
I don't need another web server for webpages, SME has plenty of i-bays for that!
Now if I ever take the time to install VMware then these could be virtual machines, but would still need forward ports or figure out how to make
http://email.mydomain.com point to my virtual server...
Ryan