Refers to a situation where any less than the whole item is worse than having nothing at all. The idea is that while it's obvious that sometimes doing (or having) something half-assed is worse than doing it properly, sometimes doing it half-assed is worse than not doing it at all.
Me: Hi Jim, do you like kittens?
Jim: Yeah, kittens are awesome!
Me: Great, well how about I just give you half a cat for now.
Jim: Oh, umm, never mind.