You are right, but there is probably some misunderstanding. That the personal considerations should be ignored when assessing probability of an idea, and that one shouldn't express collective agreement with ideas based on their author, are different suggestions. You argue against the former while I was stating the latter.

It's important to take into account the context. When an idea X is being questioned, saying "I agree with X, because a very trustworthy person Y agrees with X" is fine with me, although it isn't the best sort of argument one could provide. Starting the discussion "I agree with X1, X2, X3, ... Xn", on the other hand, makes any reasonable debate almost impossible, since it is not practical to argue n distinct ideas at once.