Any user who knows enough to know what he is doing on that page already knows about it.

The point of highlighting the URL with a single click isn't just to replace it with something else, but to copy it fast, and all kinds of users, including true power users, copy URLs much more often than edit them.

I'm sure that's the rationale, but 'true power users' likely hit meta-l to select the whole url anyhow, instead of using the mouse...