Somehow, various players think that showing diplomatic modifiers allows us to manipulate the AI and hiding those modifiers takes that manipulation away. However, that's not the case. It has to do with the diplomatic engine, the diplomatic AI behind the conversations that you have with the AI or the modifiers which are shown on the screen. I know that the modifiers in civ4 allowed a player to manipulate the AI, but the diplomatic engine doesn't have to work that way even if numbers are shown.
For instance, if a friendly AI at +10 diplomatic rating (modified Civ4 like model) would declare war on you based on a complicated formula taking into account this diplomatic rating but also its position in the world, its victory aims, the resources it could acquire in a war with you, your relative military strength, other AI's which could be bought into a war with you, both of your military technology levels and various technological breakthroughs which might be about to occur, the border garrisons and border terrain and maybe even more modifiers, then I could see the odds of declaring war within 10 turns be a chance from 0% to 99%. You couldn't rely purely on that simple friendly status or that +10 modifier that no war would occur. It would be a factor, but only one of many. And thus diplomacy wouldn't be very exploitable. You'd have to consider many factors and wouldn't know the exact likeliness of the AI declaring war on you especially since you don't know its victory strategy.
On the other hand, a diplomatic model where the AI would show no diplomatic modifier, but where a big smile on the AI's leader face in diplomatic talks would guarantee that this civilisation would never declare war on you would be a very exploitable model.
Similarly, if gifting the AI a border city of size 5 would grant you a diplomatic plus of +1 to +3 (randomly determined) and gifting it a border city of size 10 would grant you a diplomatic bonus of +2 to +4 (randomly determined), then you wouldn't know the exact results of such actions and couldn't exactly determine the AI's relations towards you. The smaller city might yield a better random diplomatic result than the bigger one.
On the other hand, a diplomatic model where the AI shows no diplomatic modifiers but gifting 100 gold guarantees that the AI's leader face becomes a big smile for 10 turns ensuring no military action against you during that period would be a very exploitable model
For me, showing diplomatic modifiers would be an easy to use value which helps me understand the relative value of various diplomatic actions that I take. It will show me whether the AI prefers 100 gold or a size 2 city. It will show me whether the AI dislikes me closing my borders more or whether it dislikes me trading with some other AI more. It gives a uniform valuation system between all the various diplomatic actions that I take which is easy to see at a glance without long diplomatic talks. I'll no doubt like to see the new animations of the leaders in civ5 at first, but after a few games I'll probably just want to know how they feel about me without a lengthy conversation.