Because it removes many obvious tells of a deliberate key collision targeting a specific key, and thus is harder to detect. For example, pgp.mit.edu and Enigma

Because it removes many obvious tells of a deliberate key collision targeting a specific key, and thus is harder to detect. For example, pgp.mit.edu and Enigmail would currently output information for both keys that would be almost identical per 2014-08-05, the day evil32 apparently generated the keys. I say "almost" only because they didn't set the correct timestamps, and apparently did not duplicate all UIDs -- but they easily could have. The diligent PGP user will of course not fall into such a trap, but an inexperienced user easily might, and there are many of them.