...since most of the stuff I kick out into the world is pretty bite-sized, but most of my contributors do actually go ahead and read the code to get started.
I did the same when I kicked in the initial PostgreSQL compatibility for TWIG (does this thing do HTML? Let's find out: http://twig.screwdriver.net/) I read the code (not all of it, just the core stuff, main includes and the few modules I needed to deal with) in order to pick up enough context to do what I needed, which was make the PostgreSQL interface actually work so I didn't have to run two SQL databases on one already-busy server.
Most of the other contributing I do to other projects is "surface" enough to not need much reading, since these days I'm usually too busy being a technician to have time to program (dammit!).