Exactly. MM does re-routing on the fly if you've got an active internet connection. Tried it and it worked (kinda slow, but worked).

About the internal Map application. It's free and for a company who usually does commercial program, that's already too much of a give away.
The map application does, for free, what it was programmed to do: tell you where you are and can tell you what's around you.
I've found out that the 2D mode is enough for this (and can zoom out more than 3D view).
2D is also faster in updating the display (3D is slower than my car in turning, but the voice looked always on par with my driving, but this can confuse you when driving in a city like Rome).
I use both applications depending on my needs and internet availability.
If I can plan the trip, I try to use Maemo Mapper preloaded with the maps in different zooms (and I make sure my cellphone is charged).
I use the Map application in all other situations.
It was usefult few evenings ago while looking for an address and didn't have a connection on the phone (roaming).

So I suggest people to have both programs as a workaround for the bad implementation of the Map application.