Hi Toby,
Whilst looking at motorway exits on my Legend recently, I noticed that in "Find/Exits" the context menu lets you choose which map to use for exits (unlike all other searches). By default, "choose best maps" is selected; I don't know how it makes this choice - perhaps the map has some sort of resolution/accuracy field somewhere, and ours isn't marked as being more accurate than the base map?
I know it's a bit late to add to this old discussion, but perhaps it might help.
Yes, how does it decide which is the "best map" as far as exits are concerned? It's a mystery to me. I could understand it using the basemap exits if the OSM exits were not usable at all but when you tell the gps to use the OSM map for exit finding, it can show them. Grrr! Cheers, Mark