BobS: I believe ZIL is based on MDL, but with fewer instructions.
As far as I know, decyphering just the ZIL code would not teach you everything about how Infocom games worked as they are mostly data with the Z-machine interpreter doing a lot of the actual processing.
I'm not 100% sure, but I think the parser is built into the Z-machine interpreter (which is different for each platform).
Lainz: I'm not really interested in creating point and click graphical adventures, as my artistic skills are virtually non-existent and making them cross platform is more complicated.
Text adventures are also more practical for lone developers.