My cities don't share diamonds because they're in different markets, but I don't have a problem with other people playing in multiple worlds to farm diamonds. If they keep their cities to continue to acquire more diamonds, then they've earned it from playing. If they sign up for the 100 freebie diamonds and abandon their cities, then that's only 700 diamonds at most, which doesn't stretch very far.
The premise of "fairness" in such an open ended game is also a ruse. The only times it might count or matters is if you care about rank, whether that's overall ranking or tourney ranking, and for leagues in events. For both rankings, you can be a week old city competing against a 5yr old city, so how was that ever fair, even not accounting for shared diamonds? For tourney rankings, they come by every week. It's not sustainable to be able to maintain an advantage from spending diamonds and 700 diamonds there certainly won't do it. For global rankings, that might only get you one more plot so congratulations...you have the ability to spam more buildings that require working pop than most. *clap clap clap* For events, at best, that'll get you some extra essence for ONE event. However, since the denominations are 500 diamonds and 1200 diamonds to spend, you'd only get the 500 diamond purchase with the 700 diamonds from multiple worlds. That nets you...100 bonus essence, which prob isn't a game changer to let you jump leagues. If it did, it's only good for one event.
A better diamond deal?
FORUM CONTEST! Just five minutes of your time could net you 500
! You won't leave any neighborhoods with a husk of a city like if you abandon a city. You don't have to continually play/maintain another city. And you get to root for Archibald Barisol!