I agree with what David has to say; I too believe that there are a lot more users expecting the minibuffer prompt to be intangible.
For now, I need to add the below to my emacs config (as I don't set the customize variables using the GUI approach, and I would like to simply append stuff to whatever the default value is):
(let (;; (get ..) -> ((quote (read-only t face minibuffer-prompt)))
;; (car (get ..)) -> (quote (read-only t face minibuffer-prompt))
;; (cdr (car (get ..))) -> ((read-only t face minibuffer-prompt))
;; (car (cdr (car (get ..)))) -> (read-only t face minibuffer-prompt)
nil nil "Make the minibuffer prompt intangible.")))
It is helpful that the minibuffer-setup-hook is updated automatically. But I would also vote for '(cursor-intangible t) to be part of the default value of minibuffer-prompt-properties. That way majority of the users would see the minibuffer prompt behaving as they expect.