Linux.com reviewed Multi Distro which puts nine distributions on one CDROM effectively converting it to a live CD.
There is no apparent reasons why those distros have been chosen other than the fact that they are small enough to fit on a CDROM.
With the advent of the DVD (or Dual Layer DVDs), one can envision cramming more than a few Linux distributions on optical media in a bid to test or allow testing of as many options as possible.
More and more computer systems come with Dual layer capable DVD readers and/or writers. Nautopia is another excellent tool that allows you to make your own Live DVD or CD.
Of course, the advantages of using Live DVD are numerous: DVD offers a larger capacity (Dual layer DVD have up to 15 times the capacity of a CDROM), hence more distros to play with. DVD is speedier than CDROM (a 16x DVD transfers data at a sustained rate of 21.6mbps while a 52x CDROM does 7.8mbps). And apart from the price of the media and your own time, there's no other cost involved.
The only complaint that I can find is that duplicate software can be found in most packages (open office, mozilla, firefox) and that it would be a good idea to have a single folder or repository for all of them.