SPF Issue

I set up my SPF record with the information listed in the wiki

This matches the information generated by openspf.org wizard
I then noticed that an email generated by one user in my domain sent to another user in the same domain was showing an SPF_SOFTFAIL in the headers.
Further investigation with DH support indicates that the IP address of the PC used to generate the email is not in the SPF record and that is what is failing.
The IP addresses for users of my domain are not fixed. They use various ISPs and these ISPs use DHCP so the IP addresses will vary
That being the case, I can see the advantage of SPF for a corporate email, and for those who have an email address belonging to their ISP.
But SPF does not seem to be a lot of use for people like me and (I would assume) anyone who runs email from a DH hosted domain.
Anyone else hit this issue or can provide advice for me?