Kevin Werbach, internet gadfly and all-around nice guy, wrote a student Note in the Harvard Law Review entitled Looking It Up: The Supreme Court's Use of Dictionaries in Statutory and Constitutional Interpretation. Since then, Eugene Volokh's discovered that it's become one of the most-cited Notes he could find, with over 100 academic citations. Especially impressive since one of its core arguments is against blindly accepting the authority of dictionaries. An outline:
This Paper argues that the Supreme Court should exercise greater sensitivity in its use of dictionaries. Part I demonstrates the increased prominence of dictionaries in Supreme Court opinions during the last several years. This shift is too substantial to ignore or dismiss as a coincidence; some underlying factors must explain the trend. Part II therefore situates the Court's use of dictionaries within a broader context of changed attitudes toward statutory interpretation and the role of judges. Dictionaries are not ideal tools; they provide a range of definitions that bear an imperfect relationship to context and meaning. The choice of the dictionary as an interpretive tool requires substantive decisions by judges, and introduces the antecedent assumptions of dictionary editors into the legal process. Part III suggests that these and other considerations gravely limit the value of dictionaries to statutory and constitutional interpretation, and that the Court's current unselfconscious attitude towards the reference books greatly exacerbates these problems. The paper concludes in Part IV with suggestions for more rigorous and more appropriate use of dictionaries.
I love dictionaries and the people who make them, but I'm glad that are strong arguments against their being pressed into duty as legal references.
I think you'll find most lexicographers will agree with you. We tend to stress that someone who needs a dictionary professionally really needs dictionaries. They're too different from each other: they have different tendencies as far as prescriptivism and descriptivism, they have different inclusion policies, they may be based on completely different historical works (since few dictionaries are created from scratch, but instead build of off earlier lexicography), they may show biases for or against slang, dialect, or colloquialisms, and they may be written to different reading levels.
Although all dictionaries have mistakes, such as errors of inclusion or exclusion, definitions that are too broad or too narrow, and too often in having etymologies made of wild-ass guesses, some dictionaries are just bad. I won't name them, but in the paper linked above, there are a few used by justices of the Supreme Court that weren't considered good even the day they were published.
But more importantly I'm particularly bothered by the number of several-hundred-year-old dictionaries that were cited. I can't know without examining the proceedings, but I suspect the older dictionaries are perceived as having more authority than newer ones, or where the old definitions differ with the new, the old ones are seen as the "original" or "best" meanings, which is little more than a common etymological fallacy. Of course, if one is examining historical documents and trying to suss out the meaning of a word on a document signed in 1780, it may make sense to look back to see what kinds of dictionaries might have been used by the document's composer.