Aluminium cube - fan and hotend mount - is it worth it?
I think its only advantage is that it serves to move money from the folks who buy it to the folks that sell it, and that's an advantage purely for the folks that sell it.

"Additional heatsink surface area" is quite doubtful - it would have poor coupling to the actual heatsink. If the actual heatsink is correctly designed, there's no need for additional area, anyway. This design clearly does limit airflow to the top and bottom fins - the bottom one, in particular, is not a good place to limit airflow for best function, and yet it's severely occluded by the design of this block.

Save your money.

Edit - I think the plastic one shown is also less than ideal - the close coupling to the edges of the heatsink reduces the effectiveness of the heatsink, which would otherwise be dumping heat into the airflow past those edges. A better design (IMHO, gut engineering) would be spaced off the ends/edges of the fins about as much as the space between fins, not touching them. Axial fans perform poorly with backpressure, and that would also reduce backpressure.