The ARB should care. Many of these large applications that Groovounet mentioned helped carry OpenGL through its middle years. Simply dropping support for them would be a bad political move. It is unfortunate that these apps have created a lot of inertia, but I believe the fact that OpenGL is still around makes up for itWho cares about a couple of antique software that uses the old-school OpenGL?
These apps tend to have huge amounts of GL code in them, and there are instances where "updating" to the core profile simply has no positive performance impact. Forcing core on them would just make for busy work, which doesn't make customers or marketing departments very happy.
The compatibility profile is useful for migrating an older GL application to more modern OpenGL. It allows for a more gradual transition of GL code. Otherwise, you'd be stuck making the switch to core, wading through tons of old GL code and hoping it all works out in the end. For a new application, it would make more sense to start out using the core profile, though (or at least adhere to its tenets).Trash the compatibility profile and rewrite the specification, straight to the metal...version 4.5?
There is certainly no one telling driver developers that they must support the compatibility profile - it's optional. Developers that have had GL implementations for years will likely continue offering the Compatibility profile (AMD, Nvidia). Other developers have retired it, like Apple.
I would like to see DSA as part of the core profile only. Since it's a fairly major API shift, it might possibly be the only change in a new GL version. Those people using the compatibility profile already have all their code built around the old bind-to-modify paradigm, so I can't imagine it'd be a great loss to not have in the compatibility profile.
If opaque handles truly improve performance by a decent margin, I say bite the bullet and reissue all the newer GL3/GL4 direct-access functions with opaque handles, and do it all at once when implementing DSA. I would also say deprecate the int-versions, but I guess deprecation is passe nowadays, so they'd likely have to stay too.