This statement of R Shimon has two possible meanings. One is that joining is worse than idolatry. But then there would be a question from the verse he brings as a proof "Only to God."
Or he means idolatry and joining are equal but they did not accept the Gold Calf but just desired it. But then we have a question from the verse where they said "these are your gods."
The reason I think that people don't like to her this is because "joining" is a delicate point.
For one thing sometimes a person is following some great leader--and in fact that leader is great, or sometimes there are following someone who is not great. But in both cases they are adding something to God. So when I mention this particular small paragraph it makes them uncomfortable.