Except the rest of the party is thinking the same thing, or at least thinking youre thinking that and expecting you to try to backstab them to benefit yourself. so you won't really get the chance, because everyone is watching their own backs.
They aren't going to just *assume* youre trustworthy if you don't have to be.
And they all come up with contingency plans. They spend much of their cash on their own survival gear. Potions, magic items, whatever. They don't just upgrade to the newest weapon their class uses. They are prepared for that ogre. and while they may not be able to take it out on their own, they likely have thought it out in advance and have ways to escape.
Maybe when you decide to abandon the cleric to his fate, he casts darkness on you and then leaves before it wears off. or hold person while he leaves you to the ogre, or casts animate dead and tells them to attack you AND the ogre. or maybe you suddenly find yourself in a Geas to defend the cleric, or a geas to kill the ogre.
Every class has things they can do to be nasty. my issue is that the cleric can be nasty like a mage while crushing things like a fighter.
players not being able to guarantee that they will have help from their 'friends' means theyll play smarter, and pay more attention to whats going on around them.
and even IF the party has teamwork, they shouldnt assume someone will fill a certain role. Just because I'm a cleric doesn't mean I'll take healing. just like if I'm a rogue I might not take disable device. and whether or not I take those things has nothing to do with the rest of the party, those are independent decisions that have nothing to do with them.
most of your argument is more of a "I dont like evil PCs" point of view. whether or not you like Evil PCs really has nothing to with whether a class is balanced. deciding to use your abilities differently shouldnt make you greatly more powerful than any other way of making them.