Aside from shallow, uncritical notes, consider that annotating in a margin is evidence that a reader is actively engaging with a text.

Very often when I write in margins it's to question or argue against what's written (e.g. "really? what about ...", or "no, contradicts ...") which, to me, seems in line with the dialectic process Socrates promoted. Multiple sides, truth-seeking.

Sure, having a spacious notebook on the side is even better. But it's the same goal, isn't it? Engage more deeply with a text and encourage questioning as you read it, while it's still fresh.