It's cuz what she can do is search of ANY rebel with X mana cost and put it into play by just tapping her and paying X. This gave her so much synergy with rebels that every rebel deck ran 4. It came down to whoever playing her first wins the game, that's all. And her casting cost is like 4 or something, which is a deal (since there are rebels that make you pay 5 to search of a rebel with mana cost of 4 or lower and put it into play). this rebel deck is also immune to wraths and stuff due to the fact that there is some rebel that brings rebels back (and the cycle begins anew). So now we just patriach's bidding, then they used rebels. lol
btw, mercenaries had a push in Masques too, but they were generally horrific.