I use the cup and it’s adapter a fair amount for smoothies, to grind up small amounts of nuts and to make chermoula type sauces and seasonings.
For me, the adapter is BIG, heavy and awkward to manage to get on the cup, screw down tight enough that it doesn’t leak and then reverse the process. However, that doesn’t stop me from using it!
I wanted the smaller size because I feel that with small amounts, too much is wasted with the blender - and frankly, it’s nice to make my smoothie and drink it out of the same container.