The situation is better with libvirt, so an additional disk may be an option in this case.
Yeah, it's possible we could get a second disk to work, but that requires additional config management changes to mount it in the correct place, and keep our fingers crossed we can avoid bugs similar to those links across various dev host environments (out of my control). So I'd much prefer to have a larger disk in the base image if possible.
Regarding KB's point, I haven't tested that, but it seems possible this could lead to filling up the host machine disk -- of course that problem exists regardless of the VM disk size, it just may be more likely with a larger allowed guest disk size.