I heard from a climber, well more than one, that threading webbing though webbing is a no-no. They were talking about linking two lengths of climbing webbing together by threading through the end loops. Something about webbing to webbing wear and the webbing failing. Don't know if the same applies to hammock use.
I use the biner simply because it is much easier and quicker to click the webbing into the biner than thread 7' to 10' of webbing through the end loop and then pulling it back out again.