I’ve been pondering this and just ran into an example of why I think that Good Post and +1 Agree both have places. In this post:
I am pleased that @Ed68 is providing useful information that both continues to add to the thread’s ongoing story and potentially helps others reading. It’s definitely worth a Good Post. However, agreement is simply not in the cards—I haven’t confirmed what he’s saying, so I wouldn’t want to say that I agreed with him.
A lot of @Shamino’s longer posts fall into this category as well. They’re utterly great, and I strongly suspect that they’re correct, but when I have used the Good Post equivalent 
on them in the past, I’m saying “I think this post makes TidBITS Talk a more valuable place,” as opposed to saying “I agree that the point made here is correct or that I agree with the opinion stated here.”
To put it another way, +1 Agree requires that you either have explicit knowledge that allows you to back up what the poster is saying or, in the case of an opinion, you’re aligning yourself with their point of view. (“Backups are good” +1 Agree)
Both are positive signals, but I like the differentiation.