Why does MAMP PRO need access to my keychain?
MAMP PRO stores server account data (2 items [(S)FTP, MySQL] per account] in it. HockeyApp library & Dropbox library store 1 item each (access token for their servers).
Even if a user grants MAMP PRO access to the keychain the software can only access data it has created itself, i.e. where it has the necessary access right (see "Access" tab of the info window of a keychain entry).