Persistent Vs. Full Install Ubuntu
 

So can somebody explain me why they do a Persistent ubuntu on usb drive but not a full install?
Is it because of speed? Security? Because of the features??? or what??

Errrrrrrr, If talking external flash drive like USB or CF or SD. Read and Write cycles are excessive on full installs which will toast a Flash Drive prematurely. Especially if one lets the default installer loose in Ubuntu and it creates a /swap partition also.

persistent is usual in most live-usb distros. if you want to be able to boot any usb bootable pc and not hard install drivers for graphics, wifi, ... then it is better to run live.

A persistent live USB can boot in any pc that will boot from usb. The "persistent" part provides for updates, adding packages and customizations.
A full install will work only on the machine it is installed on (just like any other hard drive install). It is mated with that hardware with which it is installed.