I bought the developer key so that I can get automagic updates on multiple sites that I maintain. In a number of those sites, I am not the only admin who would have access to the settings area for Gravity Forms. While I'm not extremely worried that any of the other admin / developers would take my support key - if a site were to be compromised the key would be accessible to all.
Storing it as a password (both field type on screen and secured in database) would allieviate these concerns, help you ensure that those who have paid for support are (only) the ones making use of it, and be a relatively easy thing to implement I would expect. Wordpress has great reference code I think :P