Bootable USB without overwriting everything (what about GRUB?)

As everyone probably already knows: previous versions of usb-creator, like the one in Ubuntu 14.04, were able to create bootable installation media without overwriting the data already on them. They copied the relevant files into the existing filesystem, and only required that the ~1.5GB or so be free. Newer versions, like Ubuntu 16.04, always overwrite the existing filesystem with the raw ISO image.

Now, I understand that this way is more robust, and eliminates problems arising from the syslinux version becoming incompatible with future ISOs. At the same time, it's very handy to be able to use free space on a disk rather than overwriting the entire thing, especially in this day and age where the smallest USB drive I have is 32GB. If anything, I should be able to easily have ISOs of different Ubuntu versions on the drive at the same time, and the ability to select the one I want at boot. Hey, a guy can dream, right?

Well, after looking into it, it turned out to be trivial to use GRUB to do exactly that. The version on my 14.04 machine is 2.02~beta2-9ubuntu1.12. I installed that to the USB drive, copied over three Ubuntu ISOs: 16.04.1, 14.04.5, and a recent daily build of 16.10 (desktop-amd64, all three), and created a grub.cfg with a menuentry for each one. All three booted on my machine without issue into their respective live desktops.

Now my question is, has this method ever been considered for usb-creator? At first glance, it looks cleaner than the syslinux method, even: instead of extracting random files all over the target's root directory, there is just the stuff in /boot/grub and the complete .iso file(s), which can go wherever you want (I chose /boot/isos). It also doesn't seem to have any compatibility issues like syslinux did between 14.04 and 16.04, although maybe that's just a coincidence. And of course it even allows for a choice between different ISOs at boot.

So now I'm curious about two things:

1. Was there a particular reason syslinux was used in the first place instead of GRUB?

2. Is there any reason this GRUB method is not clearly superior to overwriting the target disk with the image? Would it be desirable to have a future version of usb-creator use it?