Nonsense about all good things in life and computers
You can also use it to express the Schrödinger equation and the generalized positional states of a qubit in a quantum computer (I totally looked that up on Wikipedia).
Ψ is the greatest letter around and best of all, it's mine!
You could use the letter P to express the Schrödinger equation if you really want (for example if you don't know how to type Ψ). Nobody cares... (well Physists probably do :p)Its only convention to use Ψ.
Exactly, I always use the letter k when I need π = 3.141592... I'm all for using P when you need the Schrödinger equation, doing that just makes Ψ all the more mine.