I find this to be a very interesting study in the mindset of a candidate. When I first began receiving resumes in this format, I took it as an indictment of recruiters and their moral compass.
The more I have thought about it, I now see this as the ultimate sign of honesty and integrity on behalf of a candidate. By "locking" their resume in to a .PDF - they are essentially saying "This is who I am, and I am not changing."
I appreciate this very much, and I find myself much more giddy with anticipation when a new contact presents him/herself with this type of file.