As an analogy, gold would be a worthless piece of metal, if it wasn't for its popularity. Worth is an illusory, human-defined concept and just like anything "the worthiness of something" is ecided by majority. Perhaps because those who don't subscribe to majority's value system are weeded out in the process of evolution, unless their value system offers some incredible evolutionary advantage. And by incredible I mean "everyone is dead and unable to reproduce except for those who think that karma points are bullshit".
In reality, reality is merely a construct of our minds, and all value systems are created by consensus on which hallucinations are advantageous and which are disadvantageous. Thus, if we decide by unspoken consensus that karma points are worthy of construction of smart comments then karma points are valuable.