They would be considered to be a market amongst themselves, however it would be much smaller then the "real" market for bitcoin and what is traded in exchange for bitcoin. The value of each additional participant in a marketplace is exponentially more valuable to the marketplace then the last additional participant; thus when a very large percentage of the "market" is using the "other" version of Bitcoin the total value of the market will be much less (in terms of proportion) then the other bitcoin market. This will cause participants to go "back" to the "real" bitcoin which will cause the other market to continue to decline in value until it goes to (near) zero
This is one scenario. I have already detailed another scenario, and there are more hypothetical scenarios we could discuss as well with different expected outcomes. The point is in none of these scenarios the value will be
worthless. Additionally, while you are correct that the network participant size is a crucial factor into determining the value of a currency it isn't the only factor. Bitcoin could have properties , even without a large userbase, that make it very valuable to internal and external users.
I.E... If it became the de-facto dark market coin and maintained that lead because initial branding
or if some group of vendors had products or services for they only accepted in Bitcoin currency.
then the newly forked code will be worthless as will the coins that any miner mines using the unaccepted code.
The flaw in this statement lies in the fact that as long as someone values a token that attributes value to that data and thus it is no longer worthless. All alts, even failed ones have values above nothing as long as 1 user assigns such value. Before bitcoin was being traded for fiat, goods, or services it still had value as people wasted time, electricity, and hard drive space to maintain that data. Value doesn't come into existence by decree of some regulator or the fact that it needs to be traded with fiat currencies.
I think the whole argument some claim that bitcoin will either go to nothing or be worth a lot is silly. If you lose faith in Bitcoin as originally principled by Satoshi there will be plenty of ideologues that buy your coins for cheap even for sentimental reasons , but more likely to be used on the black and grey markets. Confederate currency is still worth money and it isn't being accepted anywhere for payment -
http://www.ebay.com/sch/Confederate-Currency-/3414/i.html