You are both right and wrong here. The current “layer” feature of SketchUp is very powerful and useful. I don’t want it to change and I don’t think any other of the advanced SketchUp users wants either. It is very much properly working for exactly what it is intended for.
However, SketchUp “layers” just isn’t the same feature that illustrator, photoshop, paint dot NET, Inkscape etc refer to as layers. SketchUp layers are global tags that you assign to object to control their visibility. These other programs use layers to separate the document into sections that can be safely worked with without affecting the others. This functionality is in SketchUp, as you say, implemented as groups.
While I don’t want the SketchUp “layer” feature itself to change, I’d very much want it to be renamed so people stop mixing it up with layers. I agree importers should translate layers to groups, and think the SketchUp “layer” should be assigned to the group as a whole, rather than individual edges and faces. This would also solve most of the undesired geometry merging on import.