-- do people who purchase these prototypes generally do so to actually play the game, or just because they want a rare item in their collection?
In regards to the early beta game that is being discussed here, if the data remains undumped, then the answer would be to play the game. If the data has already been dumped, then it would be just to have a rare game in their collection.
Here is an explanation that I feel best categorizes those of us who have a curious interest in prototype games.
-- early beta builds of released games --
these are often works in progress of games that were being developed and eventually saw release. they can have minor differences or major differences then the final build of the officially released games.
-- final builds of released games --
these were more often then not the versions that most video game magazines were sent for review. they play exactly the same as the released versions, but are usually on eproms & special circuit boards.
-- protos of unreleased games --
these can be at any stage of development from betas in very early stages, to sometimes unplayable, extremely buggy, without sound, etc. or can also be 100% complete and fully working, but were ultimately canceled.
Every collector has their own opinions & reasons for what they collect & why.