I’ve heard that the famous "Chazon Ish," Rabbi Avraham Yeshaya Karelitz, stood up for people with Down syndrome and the like. If this is true, I understand it as follows:
Each person is given faculties. These faculties allow him to reach his potential. The greater gifts and faculties a person is given, the more he is obligated to achieve. A person is created far away from a goal but given the tools to get there.
A person born with very limited faculties obviously has a much smaller goal to achieve. So, in essence, the person of limited capabilities is created closer to his state of personal perfection.
Such a person may even be a reincarnation of a great tzaddik or tzaddeket (righteous person) who achieved near perfection the first time around, but needs to come back to this world just to "tie up some loose ends." Such a person is worthy of respect.