The pros and cons of gangbanging

How's that for a provocative title?    Actually, this is a post about TV comedy writing.  Sorry.  It stems from a Friday Question that's worth an entire post. 

Dana Gabbard asks:

Recently you noted that while in the past show staffs were small and freelance writers were a source of most scripts (at least first drafts) now most shows are room written. Can you describe why? What are the pluses