There seems to be a lot of heated debate about collections of short stories. I think that many new authors or just authors new to writing short stories often try to overcompensate with collections.
Thanks to the publishing industry we have the idea in our heads that more words equals more value. So I think a lot of writers feel guilty about charging the same for a short story as they do a novel. So they lessen the guilt by just packing together a ton of stories into what is essentially a novel-length collection. So everyone is happy, right?
There is a certain validity to this thinking. And I do think that a short story writer should provide readers with as many options as possible. However, for the most part, I feel like there are two HUGE issues that are overlooked when a writer approaches short stories in the manner I just described.
The first problem is an artistic one. When a writer approaches short stories with the intention of writing as many as they can to put together a novel-length collection I feel that not as much effort is put into each story. The focus becomes quantity rather than quality. For a literary genre that emphasizes choice, spartan wording, this is really inexcusable in my mind. You lose the "story" and just end up writing something that's "short."
The second problem is the idea that by having a novel-length collection you will suddenly appeal to novel-only readers. This is flawed logic. If someone is looking for a new novel to read they are not going to think "oh well, this short story collection is really long so it'll do as a replacement." I like short stories and even I don't do that. If I'm in the mood for a novel, I'm buying a novel. If I want a short story, that's what I'm looking for. Two separate shopping experiences.
Your audience should be those who enjoy short stories. Since this is your target audience, effort should be put into each short story. The goal is appeal to customers that go out of their way to find short stories. Not the ones that are maybe willing to give them a try if they stumble across a collection that's long enough.