Is the scratch test for environmental allergies accurate and useful?

Incredibly. Skin prick testing is the most accurate way to test for environmental allergies. Blood testing is the 2nd most accurate although a far cry from skin testing. All other non-approved forms of testing............Well, i simply state, "let the buyer beware!" there is a reason insurance doesn't cover the non-approved methods...
No. This is the original technique first described by charles blakley in 1873. Its disadvantages include false-postives, more painful than the prick test, and not as reproducible as intradermal skin test. Because of these and other drawbacks this form of testing is no longer recommended as a diagnostic procedure.