> you don't need fat binaries to run 32 bit binaries on 64 bit systems,
Of course not. It just makes it a more difficult and irritating for users and developers and distribution makers to support mutli-arch and play games with having multiple locations and packages for the same pieces of software.
There is a reason I run 64bit kernel with 32bit userland on my Linux systems nowadays.. I tried running 64bit only and things like that, but it's a PITA to do that in Linux while 64bit application support is trivial (for end users) in OS X...
> you just need the right libraries on the system, and a fat binary doesn't help you there (if you are on a 64 bit system but only have 32 bit versions of some library that the app needs, should you run the 32 bit version?)
Well if I only have a 32bit-only version of a library (instead of the vastly preferable 32/64 fat binary library) then that would presume that only 32bit versions of that library exists.
Therefore a 64bit version (or a 32bit/64bit 'Fat binary' version) of a application that depends on that library would be impossible to have in the first place... right?
Either way it's not really any different then what we have to deal with it now, but with Fat binary support it would be handled intelligently by the system were as right now it requires a lot of manual intervention and a significant technical understanding on the part of end users to deal with these sort of compatibility issues.