It's not as trivial as saying "serve this file" - the file has to be understood by the 323 first.
For example... Take into consideration the 323 doesn't share via UPnP .AAC music files, but it does share .mp3 files. Let's also assume someone tries to get smart and renames song.aac to song.mp3 in order to get around it.
If the 323 didn't have built-in checking, and served up this file to your PS3 - it could cause unexpected things to happen including frustration and possibly corruption or undesired operation of the PS3 and possibly the 323 itself.
The UPnP server in the 323 looks for known file types by extension first, then looks to see if the file is valid, then reads the artist, song title, genre, and other attributes, depending on the file it's presently scanning. In other words it has to have logic or the 323 acts as a "non-intelligent" server and spit out whatever requested whether or not the data is valid/good/appropriate. At least the way D-Link has it setup, there is an expectation and a stable product with a reasonable user experience.