I think that you're trying to say that user-defined currencies do not create additional demand for Mastercoin, and thus do not increase its value.
It is true, to a certain extent. (They make Mastercoin more useful no more than they make Bitcoin more useful.)
Yes, this is precisely what I am getting at. SatoshiDice used Bitcoin to spam messages. Mastercoin appears to also exploit Bitcoin with no benefit to the transport currency.
However, you're missing that demand for mastercoins can come from other directions: mastercoin provides features which work only for mastercoin-based currencies, and mastercoin is probably the biggest mastercoin-based currency, so people will use it.
Particularly, escrow-based currencies and CFDs: when/if these features will be in use, you'll need some mastercoins to use as an escrow/collateral for CFD. If there is a large CFD market, (e.g. suppose it requires $1B of value in collateral), mastercoin "market cap" will also be large.
However this is no different from someone issuing his own currency using an Open Transaction (OT) server. I can get the kind of escrow functionality mastercoin can dream of today.
You seem to be missing the fact that mastercoin itself will be used as a currency. Would you rather use Mastercoin which has $200M market cap, or some shoddy "user defined coins"?
"User defined coins" is not the only feature.
That is not the point of "user defined coins", these could be company stock, bonds or other instruments that back real world assets like gold. They derive their value from the entity that issued them.
Furthermore, the Mastercoin market cap is at present not based on an efficient market. Let's see how it reacts when placed into a real exchange. Let's see how Mastercoin survives in the real world where people are always trying to game the system.
This could go all crumbling down via a double spending attack. Let's see if any exchange with a lot of BTC to lose will trust this coin in their exchange.