I don't think this is a bug. Had you said "yum install MySQL-OurDelta-shared.x86_64" then only the 64 bit version would have been installed.
The 32 bit libraries are in the repository so 32 bit binaries can be run in compatibility mode.
Many other system libraries work the same way.
For example, there are 269 packages that match lib*.i386.rpm in the Centos5 base repository (and 307 x86_64 libraries).
I don't think this is a bug. Had you said "yum install MySQL-OurDelta- shared. x86_64" then
only the 64 bit version would have been installed.
The 32 bit libraries are in the repository so 32 bit binaries can be run in compatibility mode.
Many other system libraries work the same way.
For example, there are 269 packages that match lib*.i386.rpm in the Centos5 base repository
(and 307 x86_64 libraries).