not that I would but BE myself, but if you want the most favorable seats, either buy a fare that includes seating, or if you buy BE, pay for a seat assignment in advance.

maybe you’ll get a good seat if you let UA pick it for you, but there’s, at best, equal or more likely less than that of getting a good seat last minute. Those who pay or those who buy a fare that includes it are going to pick the more desirable seats first otherwise, and sure, someone might cancel, etc., but if the goal specifically is a better seat, use one of the most guaranteed ways to get it.

put another way, if your goal is to simply save the most money - buy BE and you’ll get the luck of the draw on seats - possibly a good or ok one, or very likely not (I’d honestly assume the latter, and be happy with anything better). If your goal is to get a good seat, your best bet is at time of booking. If you’re straddling between saving money and getting a decent seat, buying BE and a seat assignment at booking is probably your best bet.