I agree with all the points of view, compiz has some major problems and dependencies.

Just an overview on the main graphic cards:

• Intel: open source drivers, compiz runs smoothly AFAIK, it could be enabled by default.
• ATI: open source drivers have some performance issue, proprietary drivers do not support AIGLX and anyway compiz has some bugs with these cards (I’ve one of those inside my laptop) thus I don’t think enabling compiz by default it’s a good choice now.
• Nvidia: open source drivers do not support 3D (I’m not talking about nouveau ’cause they are still too experimental), proprietary drivers are really good but they’re not bundled with Ubuntu thus compiz can’t be enabled by default.

I’m sorry but I can’t find a way compiz could be enabled by default… I think the right decision is to disable it until all graphic cards would be supported and bugs would be solved. I also think that, if this feature is so important for Ubuntu, Canonical should hire some programmers and sponsor the compiz development.