It was worth it for my then-5 year old! She had ok freestyle and backstroke and could swim a couple of laps but without much stamina. By the end of minis, she also had breaststroke and a racing start off the blocks (our league is not near DC and uses them). She loved practicing adjacent to the rest of the team, meeting the big kids, and wearing a team cap. Now she’s a competent 8&U and loves swimming, but more importantly she has lots of friends at the pool.
The kids for whom it was not useful were the ones who couldn’t swim a lap, didn’t have rotational breathing for free, and didn’t want to swim. There are a few parents every year who think minis=swim lessons. No matter what our club says in the requirements for minis, there’s always kids who spend the entire season shuffling hand over hand down the gutter and back and/or crying at the end of the lane.