If you really wanted, you could run RHEL with no subscription and compile your own updates from the source that they release. In practice, this is next to impossible to maintain as an individual, but it is exactly what CentOS, Scientific Linux, and other related EL distributions do. They remove the RHEL trademarked logos, compile the code released by RHEL, and make it available through a generic yum repository that doesn't require a RHEL subscription.
So, in short, RHEL doesn't cost money, support and packaged patches do. CentOS gives you binary and version compatibility of RHEL without the cost.
Enterprise. You kind of answered your own question.
> Slackware for most of my servers
Ya, you're not the target market. Also, you might be a masochist.
One market that it seems to be strong in is the defense world, since RHEL is one of the few OS' that get various certifications for safety and security.
For personal use, you can get similar stability from either CentOS or Scientific Linux, although the bug fixes will lag behind Red Hat by a few days (although some critical updates have been released only a few hours after Red Hat).
In essence, you pay for their excellent support.