There is useful expense and then there is useless expense. Now, I don't mean that in a strict sense of useful or useless, so perhaps productive and unproductive, but even these are wrong. All hobbies are useful and productive as they promote relaxation, and improve our mental state.

But, take a hobby of Hi-Fi. The person invests a lot of money and in return hears relaxing music in exchange. But, when the unit is turned off, nothing remains but the memory. With Photography, there is the creativity and fun of setup and taking a picture, there is the fun of processing and printing, and this provides relaxation. However, in the end, you have something in hand that can be shared and seen over and over again. You can make contact prints, enlargements, and derivative prints. With Hi-Fi, about all you can do is speed up the record, change the volume or tone, and perhaps get more recordings by the same artist.

So, the element of creating a product is inherent in photography which sets it apart from all other hobbies, and the money spent is "useful" or "productive" in that sense because you walk out of a session with something you made in your hand, that you can share with someone else.

I think that is the way we should look at the uniqueness of our shared hobby.