The nobility kept squabbling over land rights and titles.
The nobility, as they are wont to do, kept squabbling over land rights and titles.
This example works both with and without the "...as they are wont to do..." sub-sentence and in this particular case the punctuation is clear because the sub-sentence does not break the flow of the main one. But sometimes these subs are asides, as in this example:
The nobility, as always, kept squabbling over land rights and titles.
The sub "...as always..." clearly breaks the flow of the sentence and is meant as an aside, spoken in a different tone of voice maybe even whispered if said nobility was within earshot. The comma's separating this sub and main are not quite as clear anymore and it almost feels like it requires parenthesis instead:
The nobility (as always) kept squabbling over land rights and titles.
Since I favour long sentences, this kind of thing actually happens quite often to me. And I am never quite sure on how to arrange the punctuation. Comma's or parenthesis?