It is often the case that a binary .egg is usable and an sdist is not (when the
package requires compiling C or C++ code).  Here is an example where this is
currently happening:

http://allmydata.org/trac/tahoe/ticket/773#comment:7

Tahoe-LAFS requires zfec>=1.1.0.  There are two distributions found during build
a binary .egg of zfec v1.4.1 for the current platform and python version and an
sdist of zfec v1.4.5.  Setuptools is choosing the latter, which then fails to
build because the C/C++ compiler isn't set up on the target system.

If setuptools would prefer binaries over sdists when both satisfy the versioning
requirements, then this would be better.