Princesses are a nice addition to any team, but they are generally not invaluable to it (not counting cheat mode Karma here). As Lafate said, Bandits are usually a nice replacement for Princesses because of their huge HP pool and (generally) their skills and high base attack allows them to just "ignore" the enemy's defense (and by ignore I mean brute force through it). Valks are the alternative in case you need some MR and you don't need to kill that unit as fast as possible (if it's armored, because her attacks won't do much).

A good team of silvers can get you through many, many maps without too much trouble, but it's true that it requires more knowledge, reflexes and skill than other teams with higher rarity units in it.