A couple of curls and extensions aren't going to kill you if they're added on top of a well-designed program. Problems occur when isolation takes precedence over compound movements, but that is unlikely to occur with anyone who has the faintest idea what he is doing.
It does not follow from the fact that bodybuilders are generally unathletic and unfit by Crossfit standards that bodybuilding training will make you unathletic or unfit, as long as you're also doing your o-lifts, sprinting, and gymnastics. In other words, bicep curls aren't going to automatically undo your overhead squats and 400m sprints.
That said, I personally don't have much use for isolation movements aside from fun workouts when I feel too beat up to do anything else.