I think it's a bit unfair, oversimplifying, misleading and one-sided to say Uchihas are easily used...Take the examples you mentioned. Obito and Madara both were Uchihas themselves, they were not just used, but they themselves used others too. So Uchihas are also good at manipulating others (Itachi and Sasuke also manipulated others). And I don't think it's only because of their emotional unstability...f.e, Itachi wasn't really the emo bishing type, and note that he could keep both Obito and Danzo at bay. So you shouldn't generalize...In case of Madara or Sasuke it had much to do with revenge and the hunger for power as well, but the latter could also be said about Orochimaru, yet the guy was one of the best manipulators out there. Sure you can trace this back to some childhood trauma (f.e losing parents) but that's some oversimplified Freudian approach imo. Many characters lost their parents but it doesn't mean they were easily being manipulated (take Iruka, Naruto or Gaara - these three were also being manipulated but they had a change of heart, just like Sasuke or Obito had a change of heart ).
As for revenge, I think Nagato falls in this category too, along with Obito I think, but at the same time both of them had a genuine desire for peace deep inside.
Also, if you look at their cases individually, I think that there are stark differences that should be considered. In Obito's case, for example, Madara saved him and set him up, told him lies and brainwashed him. You can say that Madara was using Obito but you can't really blame him of being 'easily' used. Or in Sasuke's case, he was just a kid, left all alone, and Itachi planned that Sasuke would live for revenge. But you can't really say Itachi used Sasuke. As for Orochimaru, Sasuke perfectly knew that Oro wanted his body, but he was fine with that, so you can't say that Orochimaru used Sasuke so easily. Sasuke went to Orochimaru on his own, willing to lose his body as long as it meant gaining enough power to defeat Itachi, but when Sasuke realized his own strength, he used Orochimaru. He also used Karin and the others.
Note that the Uchiha's history was many a times about using each other to gain more strength (taking each other's eyes and stuff).
And let's not forget that the whoel ninja thing is about deception.
