Job Descriptions have been a very interesting topic to me. I had a quite nice one with Westinghouse 10 years ago, basically an "internal consultant" description. Then Fluor corporation took over the Hanford operating contract, and threw out the existing job descriptions. For five years, my official job title was "Civil Engineer" as that was what my undergraduate degree was in. After much pain (and nearly getting fired by my manager at the time), I got the job title changed to "Quality Engineer". Two years ago, my title was changed to "Safety Scientist" as I didn't work in QA but I did work in safety, and they were going through a big exercise to rank employees in each department by job title.
And at best, the job descriptions through these title changes have just been generic and have never matched what I did. But from what I can tell, Fluor has never been big into job descriptions. The plus side is that it does keep things flexible and you can't use the excuse "it isn't my job" or "I can't do that because I'm not allowed to".
Is this a problem? I think it was as I was overcoming hurdles to try to get my work accepted. Now that my work is well accepted in the company, it is not so much a problem. The pay has been good, and pay raises forthcoming. I have always been a "self-directed" one person work group anyway. The only hazard could come when the next set of management comes in (which inevitably happens some day), and my "status" is not documented.