Yes, a lot has changed over the years.  I've been here since before it became Renderosity.  At that time, people seldom credited anyone for their content (which used to be pretty much all free).  But then there wasn't much content available and folks typically did create all or most of their own objects, textures, etc.  I used to model objects and create textures frequently back then too.  Now I seldom do and I'm far from being alone there.  There's nothing wrong with crediting your sources if you want to and can remember were you got everything.  The purpose to the MP vendor listing is 2 fold.  1)  To make it easier to do so and 2) Yes, it does help advertise MP vendors which helps keep this site going.   As far as I know, this is all optional on your part though.  There is nothing in the MP license.txt or the TOS that says you must use the credit feature when uploading images.  It's great that you want to credit everything used in your images.  But now a days, I'd think it best to assume that the artist didn't create everything unless they specifically mention that they did or it's posted in the 3d Modeling gallery.  If you're curious about who created something or if a plugin was used, just ask the artist.  Most folks seem to be happy to tell you all about it. 

Irene-