Thats cool, at least we can both agree this comes down to the user's experience of the game. I am curious though, what sort of opportunities costs do you mean? Do you lose something by paying some attention to diplo? and in what way is it an effort to maintain diplomacy without jeopardising these potential opportunities?
Passing up good city sites to avoid the penalty for 'reckless settling' or "we covet your land." Passing up wars of expansion to avoid the warmonger hits. Passing up wonders - especially around Egypt, to avoid that hit. Loss of gold/trade options when 'friends' ask for help, and not many other opportunities to make positive in-roads without opening myself up to that.
And there's just being distracted by the 'mental clutter' of trying to remember what will and won't annoy the AI (and which AI's I actually care about not annoying vs which ones I want to keep on good terms) - which is admittedly an issue of my own playstyle/skill-level/personality.
In the end, I just find I get similar results to not-caring as I do if I try to pay attention. I get some decent trades early on before the negatives from "trying to win the game" build up too high, and when it starts to peter out I'm either in a dominant position or too far behind for "fair" trades to catch me up anyway so it doesn't matter.
So there's no real feedback to show if I am improving on that side of the game, and likewise no incentive to try improving.
The funny thing is I find one of the benefits of paying attention to diplomacy ie. reduced no. of DoW's, friendship blocs, steady trade etc. (generally maintaining the peace) is that I create more time to focus on building my empire - the one thing that a lot of people are saying CiV fails to deliver on. Surely that's a good thing, no?
Heh. The ironic part is that, for me, juggling the diplomacy creates more mental clutter that distracts my focus on building my empire that I just don't see that particular benefit. Friendship blocs are (mostly) just an excuse for us to start begging off each other instead of trading with each other. And if I get a DoW... well, the timing might not be ideal for me but it saves me the trouble of declaring on him.
Now I know this apparent failing is based upon a comparison to previous incarnations in the series and to be fair the only other Civ I've played is IV, but I've played my fair share of CiV games in which it was clear - having the soundest infrastructure by the late game was my means to victory, not militaristic domination. I get why people call CiV a wargame, but it's not like you have to play it in this manner.
There is a bit of Past History on my part as well.
My preferred strategy in Civ3, which I carry somewhat to Civ4, involved early conquest of my landmass then buckling down to semi-peaceful end-games. But in previous games, the AI was more... forgiving? of early conquest.