I tend to think accreditation is not so much a security issue as a branding/marketing/sales issue... how do you get employers to ask for it if it isn't established yet? How do you get applicants to put in the time if employers aren't asking for it?
Evaluating skill level is also tricky. Tests tend to be good at checking for basic and obscure knowledge, but not great at testing core facility with some area of knowledge. Coding is a partial exception, but I've taken a few coding tests and they're usually pretty poorly conceived!