I think you'll find that my creator x does NOT have the z screw held at the top. I've looked - there is nothing holding it :-)
My klic-n-print - essentially an unenclosed pro, Does.
The creator x makes a lot of noise, the knp almost none. Not having the z screw wobbling about must help in some way :-)
Plus it gives better stability for higher resolution prints and makes the bed that bit more stable.

Also bear in mind that you only need enclosing for standard nylon and abs. Nothing else needs it.