"In addition to the Biblical commandment of taking the four species to rejoice on Sukkot, there are also two other commandments that were fulfilled in the Holy Temple during this festival. However, these two practices are not mandated by a verse in the Scriptures; they are included in that body of custom called halacha l'moshe mi'sinai - details of religious observance that G-d taught to Moses at the Sinai Revelation. Moses subsequently related these to Joshua, and on to the Elders of Israel, and likewise throughout all the generations they were transmitted orally. These two items are the "special commandment of the willow," and the water libation, which we will discuss further on."
It is claimed, and practiced by the Pharisees, that unwritten laws exist, outside of the Torah. In the festival of Sukkot, 2 ceremonies, one of the "willows" and the other of the "water libation" have no basis in the Written Torah. However, the Rabbis believe these to have been Oral Tradition from Moses. In the days of the Sadducee High Priesthood , this led to physical violence, and according to Josephus, the civil war between King Alexander Yannai (Janneus) and the Pharisees. The Sadducees, then led by Alexander Yannai - the High Priest, - rejected the water libation, and was pelted by the Pharisees with Etrogim.
The water libation is naturalistically an interesting concept. It resembles a pagan rain prayer or sacrifice, for there to be rainfall during the upcoming winter. the only problem is, that it is not included in the Torah. And there is, unfortunately, no evidence that it was given to Moses on Mount Sinai. If it were, the practice would not have been forgotten by the serving Kohanim so easily. And there are no records in the later books of the TNK of it having been practiced.
As previously mentioned, in Devarim Ch. 30 it tells us that keeping to the written law will be rewarded and we will be loved by God.
9 And the LORD thy God will make thee over-abundant in all the
work of thy hand, in the fruit of thy body, and in the fruit of thy
cattle, and in the fruit of thy land, for good; for the LORD will again
rejoice over thee for good, as He rejoiced over thy fathers;
10 if thou shalt hearken to the voice of the LORD thy God, to
keep His commandments and His statutes which are written in this book of
the law; if thou turn unto the LORD thy God with all thy heart, and
with all thy soul.
One additional argument is brought, namely that keeping extra rabbinic laws will make us more on guard about torah laws, since the rabbinic laws seem to act as some kind of buffer zone in transgression of Torah Law.
This is a possibility, but it also carries dangers. A pre-occupation with non essential, and indeed unlawful religious rites can cause a lot of harm.