Dunno, I'm no chemist. But it is easier to eyeball min/max/derivative from a graph and compare regions. Maybe one of those properties is important.

Sure but, in my opinion, you can tell all of those in both plots. The crossmarks are not even needed.

Is your opinion an opinion of the "that's just like, my opinion man" or of the lawyer's style? Because I've yet to read a book on typography or see statistical data presentation that suggested going in without horizontal guides for the eye.

Tufte's minimalist graph is much better if it is meant to tell a story or be shown to an unsophisticated audience. But if someone wants to actually refer to the data on a regular basis some guides are a much better approach that will cut down on stupid mistakes. Eyes aren't very good at scanning over blank space without drifting.