Imposed scarcity in most forms are what I see as the problem. It's a problem if copyleft source is used without respecting licensing terms. Unless I was doing something in a larger scale business capacity, I don't have a problem with bits and pieces of what I publish to be used. It's not like I've published any major database or hosted application platform. They tend to be smaller things that solve specific technical problems, like a DB query/ORM library. In most cases I think more individuals would be learning/using small parts of it rather than the whole thing as-is.
The only thing that's really a problem I see is if you're trying to build a business around developing opensource. Even then, it's better to publish non-core pieces of it so that many eyes can use and hopefully improve the security or fix bugs/holes in it.