The 'Arabah (willow of the brook) has no beauty or smell, does not give any shade, does not produce any fruit and grows alone by the water. Passers by don't even pick it.
There is a parable in Qol Sasson about a king who had many maid-servants. Among them was one who had no beauty, and none of the men-servants wished to marry her. The king seeing this, felt compassion towards her and elevated her and made her part of his royal retinue. As a result, all the other servants became her servants. In addition, the king told his other members of the royal retinue that he would not approach them unless she was among them. This ensured her acceptance by all.
The 'Arabah is likened to the maid-servant. G-d took the 'Arabah and placed it among the regal Lulab (shoot of the palm-tree), the Ethrogh (citron) and the Hadas (myrtle) -- three plants of quality and distinction. Despite their importance, without the 'Arabah, we are told that these three are Pesulim (unfit for ritual use).
This is a great message from G-d that all His creatures are equal and we should not push anyone away or reject them, because G-d will not accept us unless we include even the "'Arabah" of His people.
(See Qol Sasson [Hakham Sasson Mordechai Moshe] Ch. 43)
