Niall,
From what I have read (and that is only what I read here) my thought was that Darius would most likely be a design consultant, at least from the perspective of Mike D and most of us who do it full time for a living. I base that on working with numerous pros, who at many times have suggested changes similar to what Darius did on THAT project, and who are more comfortable with the title of consultant.
It's an interesting question we struggle with. Is a golf course architect someone who has designed several courses and has an ever improving track record, or is it someone who got paid something for some undefined service just once somewhere? Or, 2-3 times to prove a pattern?
Sorry Jeff, our posts crossed over. On your first paragraph, why would you consult with someone who knows less than you ? 
Or to put it another way, what expertise do they have that you don't ?
On your second paragraph I'd have thought that the basic criteria would be someone who could provide the basic service competently. Note I'm referring to the service rather than the quality of the design.
Niall
Well, the first is obvious......the Owner somehow believes having a Tour Pro will help them in design or more likely marketing. As for the GCA, well he gets to say he hung with a celebrity golfer. I have learned something from each of those guys, so it was worth it. And of course, in a few cases, I wouldn't have gotten the project without them.
Behind the scenes, they always want me to add them to my professional E and O policy, which can be done, for a fee, depending on the carrier. Some won't do it, not understanding why someone with no credentials would have any responsibility for design, which is sort of what your question asks, but that is the golf world and most look the other way.
And, behind the scenes, most agents have contracts drawn up that say he is involved for limited purposes, which generally include a few site visits and adding ideas from the perspective of a golf professional, but attempting to limit the scope of their input, at least legally. And, to be honest, I can only recall one case where a Tour Pro and I battled over something, which was he wanted to put a practice bunker very much in the safety zone of an adjacent hole and practice range. When told I would write a letter saying he demanded it be put there, he backed off, sensing perhaps I knew more than he about what might get you sued.
And yes, the second criteria should be "can they provide the service competently" and in reality, it happens all the time, including my own self, starting my own business, but believing and convincing someone I was competent by virtue of seven years as an apprentice to a qualified firm doing similar services. In those cases, again, maybe the title means nothing. If you were considering minimum qualifications to join ASGCA or similar, then perhaps a pattern of existing success (which has always been our standard for new members) would still be the ability to and having provided the service successfully. Of course, as long as people are involved, I doubt we would not consider design quality.
For example, when city bids got more common and complicated back in the 1950's a few civil engineers got involved because they could draw plans to city bid standards. A few went on to be pretty good gca's but others provided great draining and irrigated golf courses with little golf merit, and typically didn't get in ASGCA or even apply (although I would have to check that history) But, they still got work because of their technical skill.
Again, titles and such are really nebulous in gca. It is what it is, LOL. At least here. I think the point I am making is that in the real world, there are contracts, standards of care, responsibilities, etc. And certainly Tour Pros are very aware that they would be considered "deep pockets" and seek to limit their liability. And in reality, I have had tour pros listed as designer by the develope, and yet, despite a two year design and construction process, never met the man. Others make a few suggestions. Some provide an over riding philosophy from "Don't make it too hard" or "I want highly contoured greens" or "I want it to look traditional" to some who really explain just how they think it should play in the prevailing wind, or what kind of shots should be required on an over 200 yard par 3, etc.
I believe, that legally, none of those above would be considered practicing golf course architecture (or design). Routing, safety standards, green specs, drainage, irrigation, environmental, permitting, etc. are probably the more accepted definition. And, you can get sued for those. I can't recall anyone getting sued for bunker placement, LOL.
So, maybe that is the clearest definition between architecture and "playing in the dirt" or consulting. If you can get sued for it, you are probably a golf course architect?