Presence signing and encryption should be separate
options. There should be easy way to turn any of these
on only when needed and there should be no need to enter
passpharase on every startup when using encryption or
presence signature (of course these would not be
available when passphrase is not entered).
Now if I would like (but I am not sure I will) to have
encrypted conversation with any of my buddies I have to
turn both signing and encryption on and enter the
passphrase even when no conversation is started.
The passphrase is needed only when encrypted message
comes to me or I want to sign my presence. There should
be also a way to wipe passphrase from memory just after
PGP is about security, so it should be very carefully
implemented and user should be able to have full
control on how his secret key is used.