I budget for not purchasing Basic Economy as if I have to pay for an assigned seat I would much rather use the AA Credit Card and get my bag checked free and then purchase a better seat than having to take a gamble with the Basic Economy seat assignment. One thing preventing me from purchasing Basic Economy is to be in the dreaded middle seat with a Customer of Size.
Another issue is if I sit towards the front on a connecting flight it will be easier to make that tight connection. I see Basic Economy fine on a SJC-LAX -SJC flight on a CRJ with a 2-2 and using the Credit Card checked luggage benefit and Priority Boarding. I would discount it at all costs and pay the extra $$$ to get an assigned seat.
BTW do the Basic Economy Passengers seated in MCE get a free drink? If so then it would be worth the gamble.