I am reading "Shree Bharadvaja-Samhita" from Shree Vaishnava-PanchaRatra this week. Verses 3.65-66 explain why Chakra-Shankha-dharana is crucial for any Vaishnava (see the screenshot below or on #flickr, #plurk, #yadisk).
A laconic translation for those, who don't know Sanskrit/Hindi:
"Shree Vishnu-Chakra (Shree Sudarshana) and Shree Vishnu-Shankha have the AgniShoma svarupa (अग्नीषोमा - agnISomA - #spokensanskrit). They Both belong to the main weapons of Bhagavan Shree Vishnu, so a vaishnava, who wears on his body Chakra and Shankha symbols actually wears all weapons of Bhagavan Shree Narayana!".