I had noticed this, too. During the pandemic while JetBlue was waiving change fees on Basic fares (through 7/20), there was REALLY no reason to book a Blue fare if planning to purchase an EMS seat. The point earnings alone were definitely not worth the cost differential.

With change fees now returning to Basic fares, I suppose that's enough of a reason (along with the boost in point earnings) to justify booking the Blue fare with an EMS seat for some customers. But, to answer the OP's question... unless you need that flexibility (or want to pay for peace of mind), there's currently no reason to pay a Blue fare if you plan top purchase an EMS seat.