I work as a consultant in the GIS area, and have some insight in the differences.

First, ArcView is designed as an "open" GIS tool, meaning that you have to get your own maps and create your own cartography. With MapPoint, you don't need this, but on the other hand, you can't do it even you want to. This I see as the most important difference, the control of the map data.

Second, ArcView file format, shape, is an open standard and can be used with other products. Most of all from within ESRI:s Arc product family which contains a lot more powerful tools that MapPoint. MapPoint is designed to get easy access to a lot of map data to display your own business data on, and do a few GIS operations on, like distance buffering, routing and such.

Conclusion: MapPoint is great to get going in the GIS area, but if you are serious in becoming a map vendor or a GIS supplier, you should really look into the heavier artillery. There are other products that compare to the Arc family, MapInfo for instance.

Hope this helps a bit.
- Niklas