According to rule 707.1:
707.1. Some cards allow spells and permanents to be face down.
Does this mean the fact that some cards allow this behaviour that any spell can be played in this way?
This theory is further reinforced by rule 707.7
707.7... Spells normally can’t be turned face up.
In addition, all morph spells I've seen have the same cost (3 waste) and they all come into play as 2/2 colourless creatures.
Finally, rule 707.11 starts:
707.11. If a face-down permanent would have an “As [this permanent] is turned face up . . .” ability...
Which suggests face-down permanents may not have such abilities...?
My thoughts are that this ability would help in cases where a player is struggling with mana flood, such that they have no other spells to cast, but could (for example) play a basic land face-down for 3 waste as a 2/2 colourless creature in a pinch.
There are other games that permit this kind of play.
However, I feel I am probably incorrect in my theory and perhaps the bit I'm misreading is rule 707.1, in so far as the "spells and permanents" mentioned are specifically those from cards that allow this ability.