Yes, certainly, as long as the language of input is MARC, then absolutely anyone who does inputting needs to be able to understand that language. My point is that we haven't created everyday-use tools that do the MARC translation in the background so people can do cataloging work without knowing MARC. My point is that we're requiring people to learn a language that they really shouldn't have to learn. Programmers should know it; managers should know it; but lower level cataloging assistants shouldn't have to know it.
Why should even managers have to know the difference between 650 $x and $v (answer: they shouldn't because $v is an abomination).
Rarer tags have higher error rates (the rarest are almost always miskeying ; do statistical analysis of tag frequencies and co-occurrences, and you'll spot all sorts of mistakes (e.g. hitting a key twice, pushing the last digit of the tag into the first indicator).
For that matter, if the only study every conducted on the, er, subject, found only about a 50% success rate for *technical services* professionals in generating paraphrases of pre-combined subject heading strings, with reference and general adults scoring worse, do they have a meaning (and in what language game)?
If collapsing two fields changes neither recall not precision when searching, does it make a sound?
If splitting a field changes recall or precision, which is it not split?
Why must we know the diameter of a cd-rom?
Is this ruler really necessary?