You might not have a manager , but you always have a Boss to a certain extent. There is always someone who is named as the companies MD/CEO , someone who has the authority to legally hire you / fire you. So therefor their opinion will always be more important.
Also the problem with having the team evaluate someones performance is that teams are vulnerable to cliqueness and groupthink and you may therfor often end up with decisions that are based more on popularity than anything else.
In theory at least a good manager should be able to step back from the inter-employee politics and make judgements with some degree of perspective.
There's always someone more invested. Management can pay lip service to flat hierarchies and all that, but at the end of the day structure emerges more or less by consensus, the same way everybody realizes the guy who always farts or something. The common-if-not-genetic "desire to be led" causes people to produce hierarchies for themselves, to produce their leaders. This isn't robotic, but it comes naturally to a lot of people, even if only to confer authority upon that person.
But the difference is that in a structure like Valves the inertia of those structures is much lower. So institutions don't stagnate into structures concerned with preserving their internal structure. Some unit starts to become irrelevant exactly when the people who are the top producers turn their attention elsewhere. The dead weight doesn't exert power to keep itself afloat.
Also the problem with having the team evaluate someones performance is that teams are vulnerable to cliqueness and groupthink and you may therfor often end up with decisions that are based more on popularity than anything else.
In theory at least a good manager should be able to step back from the inter-employee politics and make judgements with some degree of perspective.