Yes, some people change their avatar frequently.
Since you have no intention of getting on Ravelry - you just don't understand why people have many pictures, and why your idea wouldn't work for most people on Ravelry.
Each person has a virtual "notebook". Within that notebook is a section for your projects, which most people stock with pictures of their projects, along with all the pertinent info. Also within that notebook is a section for listing and displaying your stash - complete with more pictures. Another section (called a "queue") is for WIMs - and if there is a picture in the database for that/those WIMs, it's displayed along with the other info for that project.
There are other sections (not within the notebook) on Ravelry for patterns and yarns - all of which have pictures. This is why a photohost is needed.
Thousands of people around the world have joined for the community aspect, and for the fact that it's a wonderful database for projects, yarn, patterns and discussion. Right now, there are over 2000 different discussion groups people can join. Right now, this minute - 452 Ravelers are online, at other times of the day over 800 are there. Some of us are waiting and watching for when it rolls over to 1000. Ravelry is nothing like a Yahoo group, or a Usenet group - it's totally different, and is probably why so many people are joining it. ;>)
Shelagh