Aye, let's just say I have 'earned' enough diamonds to upgrade builder's hut. I did it and find it is handy to have diamonds. I will be more inclined to buy them to get the other levels.
Runescape, mmorpg they play it very smart. It is the only online game for which I ever paid. They have all that cool stuff not just p2p but also f2p. The differences is that f2p gets boring once you have reached about level 60. The first two weeks (New feature introduced about 2013 or somewhat later.) you have a free membership. They have introduced bonds which is two weeks membership which can be sold ingame, they have the wheel of fortune (daily reward) with member stuff included so if you should get a special item you first had to be a member to get it and that costs money. All in all Jagex knows how to stimulate players to buy stuff. And innogames, I have played a variety of games of inno(5 in total.) And I never even thought about buying them, why? They don't have a good campaign. Tribalwars, nothing. FoE I have forgotten what they do to attract people, the west nothing major to get nuggets, Grepolis they did most(of what I remember) by allowing people to have a taste of premium features and giving gold for inviting people. Maybe it works for normal people, but I am not normal so they better think of something smart or they will never see my money.
Note to people: I know this is an endless post but I am making a point.