What explains the unnecessary insertion of copulas before Hebrew and Yiddish participles used in English contexts?
For example, why the common formulation "he is yotze" rather than simply "he yotze"?
Sounds like it could be a case of overregularization of the copula+participle pattern found commonly in Chaza"l to express habituals (e.g. הוי מקבל את כל האדם בסבר פנים יפות). I have no historical basis for showing this spill-over to have occurred.
Perhaps it is simply a shortcut for code-switchers using a copula in a more familiar language to bear the brunt of conjugation of a verb without altering the meaning of the utterance too much. For example, it may be easier for an English speaker on the fly to come up with "they wanted to be mispalel" than "they wanted l'hispalel". In this case the efficiency gain is in the simple replacement of the string "mispalel" for any grammatical context referring to that event without needing to conjugate it. If so, then the above reasoning about overregularization could be the precedent that justifies defaulting to such a pattern.