I ran into the same thing, and in fact the offline data contains enough information to do routing/navigation, but there is no engine to actually use that data to make the drawing of the map...
In an initial step, there will be the feature to download tiles along the route:
and after that - understandably, since it's a lot more work, map drawing based on the vector data used for routing:
Once vector based maps are a reality, it will be easier to support features like:
- rotate map according to compass
- rotate map according to heading
additionally to the current
- north at the top of the display
and also 3D view...


Searching by house numbers is possible, as long as the house numbers are properly entered into www.openstreetmap.org - which is a noble task to perform for housenumbers you know of that you may want to search for...
In general, the house number situation in openstreetmap is not very good where I live, the quality of the data depends on the region and naturally what people put into the system.
This is however not something that Marble as an app can do something about...

On point 3, it probably depends on what the data format is if it's at all doable, but if doable, it will also depend on developer willingness to work on such a feature.
BTW Paid maps are perhaps constantly updated, but openstreetmap is constantly updated as well; a few weeks ago, I figured that on the N9 I'd be stuck with Nokia Ovi (Navteq) maps, so I fixed a couple of things there - that have been right on OSM for ages...
And here in Zurich there's a new tram line that's not on Navteq at all, but it's there on OSM and OBM (bus).