On the other hand "+1" and "I agree" and "Well said!" clutter the forum.
How do we solve this?
I do believe we have the a similar kind of understanding for when to up-vote. What is a 'nice contribution to the conversation' other than arguments or facts that 'I do agree with' in them being a nice contribution to the conversation.
I fear we would have to have a meta-discussion on the concepts of good and bad and how they are flawed in that there are no montains without valleys^^
The community is in many ways already an echo-chamber; that battle was lost a few years ago. Don't let it bug you.
In either case, both are accepted uses. PG has said as much:
However to me it feels a bit like discouraging alternative opinions, when they are hidden/greyed out.
Is this commonly accepted then?
It makes sense, then, that the upvote button be used for agreement.