The advantage of the DroboShare doesn't lie in speed but in flexibility: The connection between the Drobo and the DroboShare is USB - just like between the Drobo and the AirPort.
The DroboShare has two advantages over the AirPort solution:
You can connect two Drobos which will still be seen as one big volume on the network, so you get more unpartitioned space (as if you'd ever need more than the 16TB you can currently format a drobo with).
It's possible to install applications on a DroboShare - from simple stuff like an SSH daemon to the complicated tools like a iTunes Media Server (Firefly) or even a bittorrent client.
If you just want to make your Drobo network accessible though, just connecting it to the AirPort works just as well.