On RH 6.1, a system with the NFS server installed does not run mountd. This was "automatic" with RH 6.0.
It's a change in policy. You can set it up to start automatically using: chkconfig --level 345 nfs on or by using ntsysv (or another runlevel editor.)
Give me one reason I would want to install the NFS server but not run it?
'Everything' installs. One of the most reported causes of security breakins is users installing everything, and then they have large numbers of servers running that they don't know what they're for, or how to configure them sanely. If the user knows enough to configure the server, the one step extra to enable it shouldn't be too bad.