The fact that everyone ultimately wants peer-approval means we need to create participatory structures in which people are rewarded (with approval) for doing useful things. Stackoverflow is a perfect example of this. People gain karma by answering questions that other people ask.
"Everyone should have a blog" is the opposite of this. It's feel-good nonsense along the lines of "everyone has their own subjective truth and all truths are equal". If we tell everyone to publish it will become near-impossible to find voices worth hearing. It's like saying you should answer every question on stackoverflow whether or not you know the answer.
People see different facets of our identity, only God sees all of them.
In some cases our limited online presence might represent a more authentic version of our 'true selves' than we present in [the rest of] "real life".
>"it will become near-impossible to find voices worth hearing" //
Isn't the OP saying there that we all have a voice worth hearing. You're right that it would be harder to find the voices we could extract the most value from; but realistically that's probably already impossible.