Ok this is not an official item, I don't work for Fermi labs.. I work
for Red Hat but this is all just me speaking my mind.

The reason why upgrading from a beta to a final release is not
supported is that package fixes may occur that you won't see.

Say package poppler-utils-0.12.4-3.el6_0.1.x86_64 had a problem in the
build that someone notices and Connie, etal fix. They rebuild the
package but if they update the metadata (epoch, version, release) they
have to keep track of all changes so that if Red Hat comes out with an
update poppler-utils, their EVR will be different from upstreams..
this causes more problems for people trying to switch or dealing with
comparing releases. So what could end up is that the beta will have
the same numbers as the final but different code. You the user won't
see that when you do a yum update (as yum and rpm weren't really built
for this). Now there are tricks SL probably can use to get around
this, but usually these tricks come with no warranty etc.

The chances of needing these updates is also small, but it is overhead
that a tester needs to be aware of if they want to try and update from
beta-to-final.

Other reasons are similar things.. say the upstream beta had version
1.0.1-2 but it turns out that it breaks scripts or something and they
need to put in final version 1.0.0-2. The beta tester will have a
"newer" version than what was in the final release. The way around
this would be to bump up the EPOCH number, but that can cause other
issues so it is easier to say "final to final is supported, beta to
final is not."

-- 
Stephen J Smoogen.
"The core skill of innovators is error recovery, not failure avoidance."
Randy Nelson, President of Pixar University.
"Let us be kind, one to another, for most of us are fighting a hard
battle." -- Ian MacLaren