This is a serious advantage for Shadow.
Why having two types of tx instead of one ?
Look at monero blockchain size. The private tx are way bigger than the public one
By keeping the two types of tx, you can avoid to surcharge the blockchain with private ones when they are not needed.
You can also imagine a lot of applications using that possibility of having both private and public tx.
Monero blockchain size might become a huge problem if it become really successful
Block chain scaling is a problem even for Bitcoin with no private transactions.
All cryptocoins have scalability problems.
I intend to attempt to solve this.
Public and private coin spaces are a problem, because they can weaken the anonymity for the private coin space. Think about it. If you allow people to spend off to non-anonymous space, then spend back into the anonymous space then anonymity sets break down.
The CN viewkey is superior in that you can give to a trusted party without giving it to the public. Public coin spaces of SDC give the anonymity breakage to everyone.
Logic on this stuff isn't always as obvious as n00bs think it is.