There is no final application other than playing with things.
I think what your opponent is trying to say is that it is impossible to design without goals. Engineering is about creating things. If you don't know what you want to create, how can you create it? And if you don't create anything, how can you possibly learn something? Engineering without a goal is about as stupid as shooting without a target.
There are lots of people who buy Arduinos (or whatever), daughter boards, put them all together, observe motors runnings, LEDs blinking (or whatever). They may have fun doing this, but they don't really learn anything. There's no goal in what they're doing and everything is good with their designs as soon as they have fun. You're trying to assume the same attitude at the higher level. But the level doesn't matter much. If having fun is your goal then anything goes, and anything is reasonable. How can anyone possibly tell you if your design is reasonable (or not reasonable) for having fun with?
If you really want to learn something, first learn how to set goals. This is where engineering starts.