It is in our best interest to have a working, slick and easy to use wallet, and Moonstone's offering is exactly that. It might even increase user experience and adoption significantly if the development is done properly.
If it benefits other projects, then good for them. But why should we deprive ourself of the benefits it could bring us just to avoid others from benefiting from it? I would understand the concerns if they were offering it to a direct competitor to Bitshares, but it's not the case.
And you know what? It can only help Bitshares if the Moonstone wallet is used for several projects. Because all the new users that are brought in the ecosystem by the other projects (like PeerTracks) will be exposed to Bitshares because of the Moonstone wallet, thus increasing the probability of getting new Bitshares users. And the conversion will be very easy, they will already have the Moonstone wallet installed. They will only need to buy Bitshares or any assets, and bam! One conversion done.
Can someone correct me if I'm wrong?