Don't SPF/SSL also provide proof that email comes from where it says it does? As far as I can tell, DKIM's advantage is to allow for direct message forwarding (not remailing), which is a fair use case, but pretty specific. In return it introduces a fair chunk of complexity to do right, with signing and message key rotation and keeping track of who you gave keys to.

SpamAssassin assigns small positive scores for valid SPF/DKIM/whatnot headers (and larger negative for lacking either), but it's not really an effective spam deterrant. Spammers can set up their own domains that pass all the checks (although I've heard they're having good times just sending from Gmail).