There may be a reason..
..But it's not a good one. Leaving code like that means that at a glance I can't tell that it's commented out, especially if it grows beyond 2 lines. And any good editor will either be able to un-comment it for you or let you define a macro to do it in no time.
And in production code it does boil down to readability .. but readability for *everyone*, not just the author, and standardisation certainly helps with that.