After some hours of photography, you have some photos which did not exist previously. After hours of DotA you have only some improved muscle memory and so on, nothing tangible.
This heuristic would however rule out the learning of more "worthy" pure skills, jazz piano, unicycling etc.
I don't think I can formalize it easily, sadly. I guess I hate Dota because I play it even though it frustrates me most of the time, and it doesn't leave me with any lasting skill other than itself.
To me, creative skills are not inherently meaningful until practiced. Except I suppose for improving ones self-image as a "creative person," which is ultimately a self-destructive pursuit.
If one goes around "being a maker" without actually making, that distinction exists only within ones head. Seems pretty solipsistic.
Photography, for me, is similar. I take photos because I like the process and the output, not specifically so other people can see them. There's a tradeoff there, but I'm not sure I could say that playing basketball is useless, for example (although that has more health benefits than Dota).
It's a hard question. Similarly, learning a foreign language feels very useful to me, even though it doesn't produce anything at all.