Some good answers here already, but I would like to point out a case where using dates makes perfect sense: small libraries or snippets of code where the code is likely to be updated frequently but in tiny bits with no single version differing drammatically to the previous one.

In other words I'm talking about situations where there's no actual "version number" but basically a sort of quasi-daily repository dump.

When I come across this kind of stuff I usually welcome the choice as it's more useful for me to know that I'm using a relatively current script/lib rather than a specific version.