Hallo,
>> is there a special reason why..
Yes, sure.
On the USB stick with installed GRUB2, there is the folder boot and in it there are sub folders grub and iso.
In the folder iso there are the iso files LinuxMint12, LinuxMint11, LinuxMint10, Ubuntu11..., PartEdMagic and so on. In grub.cfg there are needed entries.
So, I've got with me many distributions, releases and can very easy start one of them e.g. for testing.
If a new release is out, I just download the new iso file, copy to the iso folder, make an entry in grub.cfg and without any further installation can start it for testing.
E.g. PartEdMagic (PM) is always on the latest level, end for this, very often there is a new release.
16 GB or 32 GB stick are nowadays not more so expensive, so is possible to have in a pocket plenty of different OS and use them on own or friends computers, (viruses checking, secure banking...)
A different ways are use for iso booting, I like the one from PM. PM can, if requested; save session, settings for future, even started from iso file. LM or Ubuntu can do this too, but not so easy, additional partition needed.
Have a look on the Mathias's page:
http://de.hardware-wiki.org/wiki/Multiboot-USB-Stick It will be very nice, if Plop Linux will be possible to boot from the iso file too.
Regards
swed