“I agree” would imply just that. Whatever you are saying I have no quips with and would stand right beside you in an argument.

“I don’t disagree” more implies that you have not received enough information to justify taking a stand and backing it up, or you are undecided. I may agree, but don’t tell everyone because I’m not completely sure and need to research it more. It’s not about passive or active, it’s just noncommittal.

Lastly, “agree to disagree” is just cliche for “I don’t want to discuss it anymore right now” or “I’m sick of listening to your point: I am not going to change my mind.” If someone says this to you, you’ve pushed too hard already.

But to your original point, you must admit that there are people who don’t do well with direct talk. Many of us (myself included) would rather we not dance around things, but simply speak what’s on your mind. “I don’t disagree with you” is a tactful way of dealing with the former types. It sometimes allows one to close a sale eventually even when two parties are not fully agreeing in the initial part of the discussion. The nature of some people is to shut off rational thinking and to “shut down” a stance of open-mindedness — going into full defensive mode — when their ideas are threatened. A skilled salesperson sometimes uses these very rhetorical devices you so despise to their advantage in these types of scenarios.