Should memos and briefs be written in plain English?
No, except in rare circumstances.

Memos are intended for lawyers, typically senior lawyers knowledgeable about the law. Briefs are intended for judges and their clerks and for opposing counsel—all lawyers. Documents intended for lawyers do not need to be written in plain English.

Granted, some plain-English principles work well in memos and briefs, and we all need to improve our analytical legal writing. But as a general principle, memos and briefs do not need to be written in plain English.