On a serious note, I've been doing ruby as my job for about 5 years and I work at a giant corporation with a few hundred programmers. In those 5 years I've never been bitten by someone changing a core class that breaks my code. Maybe 3 times I've been bitten by someone changing a non core class, and it usually takes me about an hour or two to resolve it.

So that's maybe 6 hours in 5 years where I've lost time due to this, but the amount of time it has saved me is far larger than this.