USB usage as main boot system

Yes, it can be, if your laptop allows booting for a usb port. you can run off the live image. however, your system on the usb won't be as up to date as on your laptop. it seems the op sys on the usb is just a pre-installed image and i tried saving udates to it, with persistence enabled, and updates wouldn't save.

which brings me to another point. if you want to save programs on the desktop and other settings, you have to have enabled persistence when you created the system on the drive.

On the usb live media created by "startup disk creator", a big file (casper-rw) is where all your changes get saved. However, a big capser-rw file itself seems to cause booting problems, no idea why, but I see many complaints,and the solution is to use the minimum size of 128M when creating it. In your case, you might need more space, so partition your 2G into a location for the read only files (830M should do 700M+128M for casper-rw), and the rest in a large partition which you will rename casper-rw after the install, and delete the file casper-rw. Seems to cause fewer problems than a large file for some reason. I have successfully updated such a disk, but the updates take a significant part of the casper-rw. Still, you will have a working, updated system on the usb stick.

i set 1 gig as persistence and have no problem booting. i'm running off a 4 gig usb drive though. i believe it's not recommended that you use more than 2 gigs. anyways, why would anyone use less than 128 mb? you can't save anything in that tiny amount.

One factor to consider: it'll be a bit slower than booting off an internal drive.

not if you have high speed usb drives.
i find that the sandisk cruzer and kingston flash drives are extremely slow.
lexar is quite fast.