Pros: Gaim would no longer store plaintext passwords in $HOME/.gaim/accounts.xml
Cons: Exactly what Sebastien said, it adds extra dependencies and forces people to enter their keyring password when running Gaim.
It would definitely have to be an optional feature, since Gaim is not a Gnome application, and that complicates things some. I'm not very familiar with the Gnome keyring daemon... but what would happen if the keyring daemon isn't running? The patch would need to make sure that Gaim falls back to doing something sensible, and possibly even warning the user. I'm particularly concerned about the case where someone switches between Gnome and non-Gnome.
I'm not opposed to adding support for this, and it sounds like other Gaim developers would be ok with it if it were done REALLY well, but the discussion on the gaim-devel mailing list about it was too short to really draw a concensus.