The meat of the question stems from several things in HP lore:
- people could get falsely accused and go to jail for nothing (Sirius Black for instance)
- people could betray other people (Potters + Pettigrew + Fidelius charm)
- people could say that they are not Death Eaters and were under influence of Imperius (Lucius Malfoy)
So, basically the vow of the following format could save both Ministry, Order of Phoenix (who whouldn't mind killing a Death Eater even if it would enforce stringent discipline in their ranks, eg taking the vow), Death Eaters (who actually used it casually (Snape-Malfoys)), bankers etc. from a world of headache:
Giver: "I swear to tell the truth regarding matter X and I swear not to conceal any facts that may pertain to the understanding and the appropriate line of questioning by Receiver"
Receiver: "I swear to hear the truth from the Giver and act accordingly and justly"
Binder: "I bind you bla-bla-bla"
Why wasn't it used as proposed? Maybe there is some rationale I'm missing.