Definitely not a downside. The only question will be whether SPF is utilized by domains where spam originates from. As the links discussion suggests, the spoofed email's "host" has to support SPF for it to be viable.
I read some Wikis a while back about SPF not being widely disseminated. That was the only reason I skipped it. If it helps, let me know so I can add it to the HOWTO?...
ISPC3 on Debian! It's great!