From: Vancouver, BC
In general, I find myself reluctant to spend MPPs on diplomacy towards Minors in the War in Europe game, playing either side. Especially in the first couple of years of the war, the opportunity cost of the MPPs per chit seems very high relative to the 5% chance of a swing per chit and the size of the swing if it happens. (I see from the patch notes that it is now 4-24% for Minors - but my guess if that 90% of the time, it will be something like 4-10% with only a 10% chance of a swing of 10-24%). Compared to investing in tech, where you can see some result every turn from each chit invested, diplomacy often seems like an expensive long shot.
Clearly winning any Minor over to your side - especially the large ones like Spain or Turkey - can have a big strategic effect on the game. But your opponent can see that risk coming from a long way off, especially if he tracks your diplomacy expenditures looking at the Reports page and sees where any swings have happened that are not explicable by campaign events. And if your opponent is investing in diplomacy, you always have the option to counter that, within chit limits. Investing in diplomacy does seem slightly more appealing for the USA, given that its diplomatic chits enjoy a 7% chance per turn of a swing. But the USA has a lot of units to buy and lot of tech to invest in, so against other uses for those MPPs, I tend to invest in those options when playing the Allies.
However, from reading this Forum, I often see references from more experienced players to investing in diplomatic chits, especially vis-a-vis Spain, but probably towards other Minors as well (perhaps Majors too). So, maybe I am underestimating the cost/benefit of diplomatic investments - hence the question in the title of this post. I would be interested to hear other players views on when and where they invest in diplomacy, especially pro-actively, and how they view the trade-offs for using MPPs this way versus investing in new units or tech.