Yeah, but I don't really rush through the campaign map. Half the time I forget I scouted for a week and then come back to the map and it pops up. Unless you're speeding through the campaign map, I don't see this being a great use for diamonds. You're essentially buying time, whereas with the expansions you are buying a bigger city, something that you can only get through diamonds (for those expansions).
I'd say the 2nd best use for diamonds is negotiating the last couple big stages in GE4, when one attempt costs upward of 100 resources. If you know that in 1 more attempt you will 100% get the answer, 10 diamonds is a decent deal for what could possibly be several failed negotiation attempts. I once failed the last stage in GE4 like 8 times and uses like 800 goods. So now I'll drop 10 diamonds if I know I'll get it in 1 more. Saves me attempts (medals) and goods to where I can negotiate more of the GE next week.