The approach with HTTP makes it accessible by other users and other computers (if you want). This makes it more easy to use it as a back-end from languages such as PHP and Java (although there are unix domain sockets available for Java). HTTP is (rather) well documented, offers good portability and ofcause is very wide spread.
The actual webserver is named Boche. The webserver and the code specific to XMMSd is very separate. When Boche is mature enough it will become a separate project to allow others using it to embed a micro webserver into their application. It uses GLib which may or may not help making it more portable.