Their concept of productive labor vs. unproductive labor was ultimately not that useful, but I really think it's interesting how critiques of classical political economy revisited the idea. Regardless of all economic activity being "necessary" in a sense, there is clearly a difference in producing new things and maintaining the system in which people function. For example (not to pick on anything, because there are a lot of fields like this), finance is clearly qualitatively different from manufacturing. That's not to say that it's wholly unnecessary, but its fundamentally unproductive nature should probably be spoken of more than it is.