Your point is valid, but not exactly correct. I don't want to know just what was decided, but why. Often I will come back to "should x be updated" again and again. Knowing the factors in the last decision is very important. More than once I've pushed to update x and had someone say "we can't for good reasons that I forget". Often I've spent weeks redoing the discussion before someone remembers/discovers why we can't do the update and we all go "whoa, I'm glad we didn't do that". Other times we discover/remember why we couldn't update and they are for reasons that no longer apply and so we do the update. Still other times we never discover any reason why we shouldn't update. Last sometimes we discover why we shouldn't have updated after it is too late and we have a mess to undo while backing out the update.

The important part is we know why a decision was made. Chat logs are not a good archive because there is too much to sort thought that isn't relevant. Better than nothing, but what is really needed is an effort to document why in a place the right people will find it when they need to know.