In my mind, "I like him but not her" does not mean exactly the same as "I like him, but not her". The comma would insist on the opposition and put emphasis on "him" and "her". It puts weight on my dislike for "her". Am I making it up?
Definitely! I use this old sentence with and without a comma:
Let's eat, Grandma! and Let's eat Grandma!
To explain, the comma can indicate action taking place in the thought line that the literate interpreter sees needing decoding. In this instance, the item of direct address, 'you' (Grandma) is left out. In other words, instead of saying, "Grandma,you and we need to eat!" the comma allows the sentence to flow smoothly through a logical contraction. Grandma knows she is the person of direct address, and nothing sinister is implied.
In the second sentence, the absence of the comma makes the word grandma the direct object, or the intended food. The speaker is suggesting to his/her listeners that they cannibalize her.