Those examples do fit: Tyrosine kinase inhibitors turn an always-on enzyme into an always-off one, when the normal behavior is for it to act as an on/off switch according to the cell's immediate environment.

http://en.wikipedia.org/wiki/Erlotinib#Side_effects

http://en.wikipedia.org/wiki/Imatinib#Tolerability_and_adver...

Once the gene mutates in a cancer cell, it's never going to work properly again (unless all the cancer cells are killed, or the gene is somehow replaced). It's just less harmful if it's disabled than if it's permanently active.

Antibiotics tend to kill good bacteria in the body, too. Most people are willing to take a week or two of GI problems and some extra servings of yogurt in exchange for survival, but still, that's some collateral damage.

The common problem is, once one part of a system in the body begins malfunctioning, it's very difficult to fix that part without affecting the surrounding system, or other parts that work similarly but aren't malfunctioning.