Both: associatedStreet and addr:interpolation are documented: http://wiki.openstreetmap.org/wiki/Relation:associatedStreet http://wiki.openstreetmap.org/wiki/Key:addr#Tags_for_interpolation_ways
"As long as we don't have a node or building outline for each house(number) along a way, it's also possible to use automatic number interpolation. For that, draw a way connecting the existing house numbers and mark it with the type of interpolation as shown in the picture above. Additional tags such as addr:street=* are still added to the objects with the addr:housenumber=* tag, the interpolation way only gets the addr:interpolation=* tag. " So in other words, addr:interpolation should connect two or more elements that are fully tagged with addr:housenumber and addr:street. So there is no improvement in adding an addr:interpolation handling to mkgmap. When removing the interpolation line you still have the single elements with the housenumber information. They are already converted to the Garmin format which automatically interpolates. There is one exception: In case an interpolation covers more than one street segment: ´| ========┴======= 1------------15 The interpolation 1 to 15 spans over the crossroads. The street have two segment so the current algorithm ignores the interpolation and would assign housenumber 1 to the left and 15 to the right segment. This could be the place where mkgmap might add two virtual housenumber nodes e.g. 9 and 11.
See statistics too: https://taginfo.openstreetmap.org/keys/addr:interpolation https://taginfo.openstreetmap.org/tags/type=associatedStreet