Like is useful because if somebody's reasoning (not necessary their personal views) reflect your own train of thought, you can just like the comment to show 'hey I see where you're coming from that makes sense'.
If you disagree, then it is on you to provide a counter-argument by deconstructing what they said.
A dislike button would just be a shitfest, akin to name-calling the messenger instead of addressing the point. 'I disagree' is not a sufficient counter argument. Either put up or shut up.
I like or I agree is sufficient if the argument you agree with stands on its own merit. But of course you can choose to go beyond a mere like and back it up with your own reasoning if you feel the like you just gave needs some validation (maybe the argument you liked was poorly worded and you can improve upon it).
In other words, don't become like reddit. That place is a trash heap.