I don't think you need MPA's for every currency or commodity. In BitBay we have already had contracts where we have longed and shorted other currencies. It's safe for both parties because of the double deposit escrow (no third party). Soon anyone that knows python can write their own smart contracts in BitBay, allowing a whole new level of complexity.
I disagree on multiple levels. I am not saying BitBay is not a possible solution to issuing MPAs, or at the very least a part of the solution, but I am just stating that other solutions are still necessary.
The most glaring issue is that there are hundreds of millions of dollars worth of cryptocurrencies traded every day. It is impossible to easily extrapolate what percentage of that is FIAT/MPA-based, but a quick glance at coinmarketcap will give the impression it is at least tens to hundreds of millions of dollars. Bitbay's 24 hour volume is $84 (that is not a typo... they did eighty four dollars in volume the past 24 hours.) There is not nearly enough liquidity in the Bitbay token to take on a sizeable portion of trading. So what if someone can enter a contract on Bitbay for any currency/commodity/etc if they cannot easily get into or out of the Bitbay ecosystem.
Also, there needs to be multiple working solutions floating around.
Some users will prefer some implementations more than others for various reasons (see nubits vs bitusd threads).
Some implementations may fail (see Nubits... although I am still not convinced Nubits won't rebound and/or a more successful Nubits 2.0 rise to replace it which hopefully learns from Nubits 1.0's mistakes).
Some implementations will have more volume than others (to small traders it won't matter, but to larger traders it will).
In theory, the more implementations there are of MPAs, the more volume that will exist as a whole, as some users/traders will reject some implementations but embrace others.
I am not familiar with Bitbay, but MPAs need to have the ability to be easily sent and received just like any other cryptocurrency, because this opens up many other applications other than simply trading. For instance, being able to transact in a FIAT-based MPA allows e-commerce sites (or decentralized markets, etc) to transact in a stable cryptocurrency. Some users may not want to be subject to the wild swings of regular non-market-pegged cryptocurrencies. Etc...
Can these Bitbay contracts be easily transferred?
I am not sure that a market place is the best way to sell and buy MPAs, as users then have to wade through the marketplace just to get to where they can trade the MPAs. A trading/exchange interface that exists solely for MPAs seems more convenient and ideal.
Admittedly, I am not well versed in what is going on with Bitbay, but I refuse to agree with your simple statement that "Bitbay is the answer". You must understand that it is hard to believe that Bitbay (mainly intended to be a decentralized marketplace) is a better solution than all four of the projects (or sub-projects) listed in the OP whose only purpose is to facilitate the creation and trading of MPAs. As far as half of the projects listed in the OP... their entire existence has been pretty much dedicated to the creation of MPAs. Even if it were true that Bitbay is the best solution, I believe that the community/ecosystem will be better off if more solutions/implementations exist rather than only one or a couple.... for multiple reasons, some of which I listed above.