Co-op games get played, but including in a single player game, yeah that's incredibly inefficient use of resources and not worth it at all. If they do co-op, it should be an expansion, stand-alone, with a separate campaign. Everybody should play Portal 2 and Lara Croft and the Guardian of Light co-op, that's some of the best gameplay you'll ever find.
The best solution is to have a separate Kickstarter for each of those various minority things that people ask for in terms of high technical costs--console port, co-op mode. Let Obsidian calculate the amount of time and money would be necessary for each (e.g. expansion built ground-up for co-op) and see where the KS goes. Then if the target audiences manage to fund either, the mechanisms are already paid for and Obsidian doesn't have to worry about losses or stripping too much content. Those are all side projects, though.
Altought i dont think there should be a kickstarter for every little thing about a project i think having a separate project for something big like adding multiplayer could work.
Let those who are interested in multiplayer fund it so that obsidian can hire the extra staff and have the extra ressource to do it.
