Some people don't see the problem of doubling the proficiency bonus, since it's been reserved to skill so far, which don't have set DC's, and there is great table variance to what a "good" skill bonus is (see recent thread about success rates).
What I do to explain to them why expertise is bad is just have them imagine what would happen if you could double your proficiency bonus for other things, like saving throws, calculating save DC's, or attack rolls. That usually gets their eyebrows shooting into the stratosphere!