ArcInfo Workstation has inherent topology. As a Geologist, you can use it to ensure appropriate spatial integrity between rock units, faults, and contacts. ArcDesktop does not, by default, empower users in this way. That explains why educated GIS users can become very frustrated with those with no concept of GIS (how many slivers, overlaps, and self-intersections caused by "dangerous" GIS users have you had to fix?). One must go through a series of steps to duplicate what Workstation provides at the get-go.
Fortunately, through the use of geodatabases and a few Arc tools, users of Desktop can ensure topologically correct geologic maps. You must define topology for lines, generate "labels" using points, and then create areas (rock units) from these parts using the Feature to Polygon tool (Arc 10 no longer provides an option to generate polygons within the right-click feature dataset context menu). If you're particularly savvy, you can export lines, points, and areas into a coverage for Workstation. Unfortunately, Workstation will no longer be supported after Arc 10. So... old-school geologists will need to move on.
If in the future you need to update only a part of the spatial data, you can clip, update lines, erase, load, generate polygons, and update (I may go into more detail later).
QGIS has yet to provide a neat way to enforce rules of topology. GRASS, however, can do it without any problems. It's basically built in as with ArcInfo Workstation. But good luck convincing organizations to use GRASS -- even if those organizations had helped to develop it!
This post is really meant to grease the wheels and convince me to begin posting regularly again. Will it work?