I'm not so sure it should be a licensor's job to somehow make the client know the difference. If I can't, as a design professional, already demonstrate how and why my work is better than an "incompetent amateur"'s, then I shouldn't be licensed anyway. For this purpose, a "license" seems redundant.
I've never had any trouble convincing potential clients that my work is superior. The trouble I have is that someone less experienced comes along and severely undercuts me on the price (Sure! I'll do a logo for $200). The potential client sees a slightly lesser quality logo, but decides he would rather save the money.
And you're right. Accreditation would not eliminate that. But part of me really hopes that it would allow the graphic design profession to be taken more seriously. And over time, potential clients would come to understand all the non-obvious work that goes into designing, therefore understanding why they should pay a fair price to a licensed designer rather than an unlicensed amateur.
(One of) The biggest problem(s) the graphic design field faces is that there are people out there who see logo creation as nothing more than matching up a font with a picture, and therefore do not understand why they should have to pay a couple of thousand dollars for an identity package.