I've taken only one of the DSST practice exams and swore I'd never do it again. I actually think I came away from it knowing less than when I started. My impression of the practice exam was no more favorable than yours for the very same reasons you stated. I've found a couple of the Peterson's CLEP practice exams to be considerably less helpful than others in a few instances, but never completely worthless like the DSST one.

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Tests Completed:
Amercian Government (CLEP)
Intro to Psychology (CLEP)
Human Growth & Development (CLEP)
Intro to Educational Psychology (CLEP)
US History I (CLEP)
US History II (CLEP)
Principals of Macroeconomics (CLEP)
Principals of Microeconomics (CLEP)
Environment & Humanity (DSST)