I have to say at only $25 for the kit, that is a damn good price for something that does what it does.
Using the cassette port card is only for power if you want to make it self contained instead of having to plug in the USB port on the main board to power is a Very very very nice idea. Plus having connectors for audio inputs to play cassette programs into it to copy to the SD. Very slick. Then on top of that still being able to plug in drives.
I'm not big on kits with my fumblefingers, but this one is tempting. I barely use my Pet because of my worry about the old old old floppy drives.