I thought it was really interesting when they gave Rarity the element of generosity, looking at how she was portrayed in the beginning but in her defense, I can also see why the writers did it. Rarity is a tad uppity, vain, and materialistic. I don't think the creators wanted these traits to totally define her so they gave her the contrasting element. I think in terms of generosity she is meant to be a "good Samaritan." In everyday situations she doesn't really show her element but when someone is in need she is willing to give up what she wants in order to help that person; like when she cut her tail for the dragon (I was actually surprised to see her do that so willingly).
I agree, I actually think that generosity is one of the hardest virtues to have; she may not be generous all the time but you gotta give her some slack.