Essential for tax declaration, registering with companies, schools, affiliation to any health system, passport, etc.
The multiple cards are to be unified in the Clave Única del Registro de Población (CURP), which isn't an ID card, but more like a National Insurance Number.
http://www.physorg.com/news/2011-01-mexico-iris-technology-id-cards.html
http://www.wnd.com/?pageId=263797
http://www.unisys.com/unisys/news/detail.jsp?id=1120000970001610095