That's a very, very astute point. Even further some people get agitated if you just say agree instead of just liking now
I think they're a little backwards. I think liking without saying anything is the lame way. Like you said, if someone starts with typing agree, it's an investment of sorts that most posters would want to follow up on a little bit. Just "liking" something is easy to forget that you ever did. And for other people there's a lot less to read, which defeats the point of the whole reason we're here.
Also come to think of it, the ones liking something may not ever post why. But the ones who dislike something are incentivized to post because they can't just hit a button. And they shouldn't be able to either, but this could make discussion more combative or one sided. Which could reduce posts overall, at least from new users who will be more anticipatory about negative feedback. The like system just makes it more natural to only ever reply to someone when you have a little criticism. I kind of like the like buttons sometimes when I don't have the brain power to think much because I'm tired, but it's easy for me to see how it reduces actual participation.
And this is a long post just to say: I agree.