But NO SPF record adds 2-3 points to your spam score (taking only a "5" to qualify) is seriously not worth NOT having one (being the default neutral). Having a "-all" and mail coming from your domain that *IS NOT* from allowed addresses (being a negative return) adds 3-5 points to spam scores (but not plus the 2-3 for no record).
This mean you can reduce mail spam scores from your real mail servers by 2-3 points, plus add 3-5 points to fakers.
DKIM and SenderID are not ideal solution anyway. DKIM is easy to implement and publish the public keys so... spammer use it *AND* SPF.
Also using the "-all" at the end of your SPF record says *ONLY THESE IP/HOSTs* are authorized to send mail for your domain.
Most places use SPF as another tick mark against depending on the outcome (neutral or negative/not-authorized). Having an SPF record proper gives YOUR mail a much better chance of getting through.
Now the Spammers that are properly signing and publishing stuff... you have a real target to go after then. They can and are seriously blacklisted, no amounts of SPF or DKIM will truly overcome the SPAM additives in the scoring engines.