I can see why it may be useful. I'm very lucky because my stoma is usually very predictable and I can change the bag during its quiet times, but from the few occasions where I have had to change it while it's active, I appreciate how that can be a big problem (if I am changing it when it's active, it's usually because it's leaked, and if it's leaked, it's usually because my stomach is very upset). When I do have to change it when it's active, I usually end up sitting in the bath to do it, and with a big wad of toilet paper or wipes held over it while I'm between bags. I can see this cup would mean less wipes/tissue wasted and probably less of a mess to clean up after.
But my biggest problem in this respect is that for various medical reasons I have a lot of trouble with fine hand movements. If the cup actually did free up your hands I think it would be a big help, but I'm not sure that it would. Maybe you put it in position before removing the bag? I think if my stoma ever does become less predictable I'd be tempted to try something like this - as with many stoma-related things, you can probably work out a way of using it that suits you if you experiment a bit. My hands are only going to get worse, so I need all the help I can get.
I'm not sure why it says you can use it to protect your stoma while bathing and showering - I have showers and baths with no bag on, water and soap don't do it any harm. Shampoo and shower gel don't seem to do it any harm. If I'm doing something like dying my hair, I wouldn't want hair dye getting on it when I'm showering it off, but then I wouldn't shower without a bag on if I'm using something like that that I don't want on my stoma. And the same would apply if I had skin problems or something and didn't want soap on it - why protect it with a cup when you can leave a bag on?