I am a fan of the spirit of Robert's Rules and once read the original (I had gotten it by mistake when I went to the library to get "Robert's Rules Newly Revised.") As president of our Residents Council I am entrusted for one year with a copy dated 1904, which we pass along to the new incoming presiding officer.
The most important principle, in my mind, is this:
"If the chairman has even the appearance of being a partisan, he loses much of his ability to control those who are on the opposite side of the question."
Further, "The chairman sometimes calls a member to the chair, and himself takes part in the debate; but this should rarely be done, and nothing can justify it in a case where much feeling is shown, and there is a liability to difficulty in preserving order."
In sum, it is important for the chair to maintain neutrality when an issue is discussed. But a savvy chair can assess the strength of both sides of an issue before a meeting is even scheduled.