I'm not sure what the support contract entails, but usually when you buy support, it's to guarantee fixes and repairs on your installation. A good sysadmin is keeping your systems running day by day and doing a lot of additional tasks that the support contract doesn't cover.
Then there are times where you're expecting one person to know DNS, DHCP, client configs, backups, email, RAID, driver issues, etc. etc. etc. and sometimes there's a slight chance that he or she will bump into something he or she isn't familiar with. It happens.
So that's where your support contract comes in. If you have a critical system that cannot go down, your contract means that your admin has access to people that know the internals of that distro and should, in theory, be able to help him or her get through a rough patch or get preferred support. On the Internet there is help available...but it may be outdated, it will definitely not be obligated, and it may be inaccurate. The vendor is someone who can be held accountable and is obligated to assist since you have a contract, and Googling through support forums and help sites when you need that web server back online NOW is not very efficient.
It comes down to your support needs and how much your uptime and system availability is worth to you. The contract may pay for itself in the first emergency you have, rather than pinning every bit of responsibility on a probably overworked and overstressed sysadmin.