I'm thinking the Prime and Bionic are for two different types of people. IIRC, Webtop is a Motorola thing.
If the Bionic main feature is its ability to be a mobile platform out the box with mouse/keyboard functionality, AKA a mobile office phone. Wouldn't that make Primes main feature ICS, and a pure Google experience. I doubt it will have Webtop (if the above is true) although it may feature HDMI out.
Did any of the other nexus devices feature docs that supported external accessories besides a TV? If not, the Bionic could still fare well, as well as the Prime as each would cater to two separate groups.