The reasonable response to that is not "so we shouldn't let anyone find out what other people are making." It's "so we should go to management, demand to know why this crappy employee is making more, and if they don't have a very good answer and won't remedy the situation, consider leaving to some place where they reward excellence."

Devils advocate: why does it matter? Does them making more than you affect you in any way except emotionally? Are management going to say "Oh! Thanks for pointing it out, here you go, have a portion of their salary!", or are they just going to stop paying that employee as much?

I believe my other comment here [0] answers that question.

However, it also speaks to what is valued at that workplace. This matters not just for the "emotional" purposes, but for the implications it has for how the workplace will evolve over time due to those values.

If you are someone for whom personal relationships, negotiating skills, and networking matter more than skill at the job, perhaps you think that a place where a lazy but well-connected developer is paid significantly more, and management refuses to change this, is ideal.

If, on the other hand, you are someone for whom competence, productivity, and ability to do the job assigned is most important...then perhaps you will find that such a workplace will become less and less friendly to you over time.

[0] https://news.ycombinator.com/item?id=28543162

Because you can use it to negotiate higher pay. I would say that’s a pretty significant affect on your life.

Having more data in a negotiation is never a bad thing, having less always is.

You're making an assumption that they will negotiate your pay up, and not correct the under performers down, or do neither. More data is useful yes, but not on a per-employee basis. If the average for your position is higher, then yes, you can use that, but if it's an individual you're unlikely to get any difference for yourself, but could negatively impact them.

Yet what if the best developer on the team was paid 3x more than everyone else, would you not also feel like shit? Unless everyone is paid exactly the same, then everyone feels like shit, which will motivate everyone to no longer care, and then your job will go away as your employer winds up as shit.

That's pretty much your fault, you go to your manager and negotiate better compensation, or leave. That have happened to me as well, and I left, never looked back. If you have 7 kids, 2 mortgages, etc... and can't afford to leave, then, maybe, that's why your employer is paying you less, because they have leverage on you. My aim is not to feel bad about somebody else's salary being higher than mine, I always aim to get as much as possible. Also,it seems the "lazy" developer, is not that lazy, as he/she/{x} at least can negotiate better compensation.