I took a look at arev and found that its \mathrm also uses operators, and \mathnormal, \mathit\mathsf all use letter families.

What's a bit pitted is that the \mathrm \ of some font packages is obviously the same font as the operators family, but it is still defined by \DeclareMathAlphabet \ instead of \ declaresymbolfontalpabet, which wastes a \fam.