One of the goals of OmniFocus 2 was to maintain database format compatibility with the existing iOS products. We didn't want folks to have to update up to three apps at the same time.
OmniFocus 2.0 will include plenty of features new to the Mac version, but not ones that would require a new database format. Multiple contexts or tags fall into that category. Hope that makes things more understandable.
In case it helps, here's
a thread from late last year which covers our current thinking with respect to the features being discussed here.