My uninformed take on the matter
On 2.4 releases, the official kernel was a bare minimum
as compared to the distributions. This meant it did
not come with a bunch of file systems and certain
device drivers that the distributions wanted in.
If I wanted XFS (SGI's file system) I either needed
to patch it myself or go with a distribution that
contained it such as Suse.
It also might have had certain memory or dispatch
code that the distribution vendor thought was wrong
for their target audience, such as RH wanting certain
large memory patches that did not come standard.
People who got their kernel from a distribution got
the stuff in it that they already wanted/needed - or
were willing to do without such as no XFS in RH.
The vanilla kernel was almost NEVER used. But it was
a known baseline for people to apply their patches to.
And people who wanted a particular feature that was not
in the vanilla kernel and not in a distribution were
hopefully savvy enough to pull the vanilla kernel and
patch it. If they were NOT savvy enough, then they had
no business determining they needed something that was
not already in it.
As of 2.6, the official kernel really isn't something
that is "locked" down. It has a LOT more stuff "standard",
such as multiple file systems, process classes for prioritization,
SE-Linux for security, (the list goes on and on).
While in the past, an even number (2.2,2.4) meant a solid
unchanging release, it does NOT in 2.6. About 6 months ago
at the kernel summit, Linux Torvalds stated that he liked
adding new things the way they currently were, and it was
up to the distributors to determine what they wanted in
their kernel, and they would lock it down for their given
customer base.
So the "vanilla" 2.6 kernel is in a much higher state of flux
as opposed to previous even kernels. I would never run a
business off the vanilla kernel.
