I, as well as most other folks, just use Wikipedia as "read-only", to look things up. I don't really envision myself being the helpful type, as you are. So there's really no reason for us to be logged in.
There is a reason for non-editors to have a username. Logged in users can change how the layout of Wikipedia appears to them through this page. Here are previews of different layouts, for non-logged in users:
The default you probably have on right now is called MonoBook.
Logged in users can also change how math equations are displayed, how large images should be displayed in pages, change the date and time edits are made in article histories from UTC to your local time, and also manipulate Wikipedia's search engine for the number of hits per page and lines per hit. You can also change whether it should underline internal links to other pages and whether Wikipedia pages should display a "table of contents."
In short, you have to sign up for Wikipedia's appearance to become very customizable.