No, you don't. There's nothing stopping you from creating `NSURLRequest` directly.
> ...wouldn't it be better to just expose all the setup functionality through simple wrapper functions?
Request serializers are not unlike any other class in Cocoa, like say NSURLSessionConfiguration, which is not often mutated beyond initial setup, but exposes properties to remain flexible and not overwhelm the user with init parameters.
What you're suggesting sounds much, much worse.
> For multipart requests, why did a protocol with appends and requiring a block with stateful appends end up making more sense than just taking an array of the parts?
Not using a protocol / builder pattern here would be awful.
You'd have to create classes for each kind of part, which gets complicated because AFNetworking handles data, files, and streams alike. So that's, like, 3 extra top-level classes. And even that doesn't really work, since it makes it really difficult to just append data to the multipart body manually, in case there was some missing functionality that AFN didn't provide. Add to that the consideration of how to specify options on the stream itself, like throttling...
You should give all of that another look—it's one of the best parts of AFNetworking.