So this was a problem with eggs too. Let's say ZODB 3.0.1 was just released. You are happily using 3.0.0, the next version is a minor upgrade, but there are no precompiled packages for 3.0.1, so your build breaks on Friday morning when you are trying to deploy.
If you find yourself in that situation then you would appreciate an easy way to use the newest available binaries rather than the newest available source,