you would need 2 files only for a tricky transition (DB rebuilds that sort of things), and return to one file once it's done, but you could wait a bit to see if there are for some bug reports before releasing the "tested" version in that situation.

And using one only file doesn't lessen the usefulness of a multiple APIversion. It's just about the rush, the mistake we make while uploading several files in a row... (you never forgot to change the folder ? ). That would be way more comfortable. Also, releasing early allow other authors to check the compatibility on PTS etc...