This is a good point, but I think there's a ready solution to that. Agreement and disagreement, by themselves, are rather superficial. Arguments, on the other hand, rationalists have more respect for. When you agree with someone, it seems that you don't have the burden to formulate an argument because, implicitly, you're referring to the first person's argument. But when you disagree with someone, you do have the burden of formulating a counterargument. So I think this is why rationalists tend to have more respect for disagreement than agreement, because disagreement requires an argument, whereas agreement doesn't need to.

But on reflection, this arrangement is fallacious. Why shouldn't agreement also require an argument? I think it may seem to add to the strength of an argument if multiple people agree that it is sound, but I don't think it does in reality. If multiple people develop the same argument independently, then the argument might be somewhat stronger; but clearly this isn't the kind of agreement we're talking about here. If I make an argument, you read my argument, and then you agree that my argument is sound, you haven't developed the same argument independently. Worse, I've just biased you towards my argument.

The better alternative is, when you agree with an argument, there should be the burden of devising a different argument that argues for the same conclusion. Of course, citing evidence also counts as an "argument". In this manner, a community of rationalists can increase the strength of a conclusion through induction; the more arguments there are for a conclusion, the stronger that conclusion is, and the better it can be relied upon.