a few day ago the iPad became full mouse support which is a step forward if you want to WORK seriously with the iPad.
Most of the time when i am on the go i have a keyboard attached to the iPad. In GarageBand this is very helpful for basic things like pressing spacebar for start/stop. While Cubasis3 has so much awesome features that Garageband hast not, this fact alone is slowing me down so much on Cubasis.
I know there was a statement that it is not designed to replace a full DAW, but keyboard support (on a base level) is so normal now on iOS that i would really urge you to reconsider.