v=spf1 +ptr - dangerous?

Hi,

I am new to this list and have my first question.

When I got everything right, "v=spf1 +ptr" means that every host with PTR 
records like
foo.fortytwo.eu.org
foo.bar.fortytwo.eu.org

and so on could send email from 
foo(_dot_)bar(_at_)fortytwo(_dot_)eu(_dot_)org(_dot_) Correct so far?

Well, spammers could easily set up foo.fortytwo.eu.org in their PTR records 
and then forge this email address. Did I miss something?
Olaf