The biggest problem with choosing the Bitshares toolkit over the existing Bitcoin blockchain, is that the Bitcoin blockchain has been running securely for years with a billion dollar market cap. We therefore know it must be very secure.
Software bugs are a fact of life, they will be in the toolkit which means that people will experience problems (lost funds, blockchain reorgs). Are these problems worth the features of the toolkit.
We also know that "Bitcoin Core" already works with all Bitcoin merchants, the entire Bitcoin community, and all developed Bitcoin software!
That's a lot of reasons to just modify the existing Bitcoin software, don't you think?