That's pretty much the kind of thing that I was thinking about. However, on second thought, regarding specifically the main motor, the benefits are much easier to see on liquid fueled rockets than on solid rockets.
On third thought, there are, apparently, schemes for throttling solid fuel rockets going back to the 1960s:
https://www.google.com/patents/US3065596https://www.google.com/patents/US3065597http://www.google.us/patents/US4357795http://www.google.cat/patents/US4483139One of the key enablers is control over the pressure in the nozzle. With 3D printing you could get quite intricate mechanisms in the interior of enclosed spaces, but I don't know if that would help. I suspect it might.
With 3D printed parts, you are also getting things like cooling channels and such built into engine parts. There are geometries that are difficult or impossible to do with other techniques. Again, harder to see how that could be relevant to solid fuel rockets, but that is literally rocket science.
https://www.youtube.com/watch?v=sJTkhXjywpQModern aerospace parts are printed using selective laser sintering or electron beam sintering. With careful control over feed grain and beam parameters you can get fully dense mechanically and thermally robust parts. Faster techniques are also coming online for less demanding metal applications:
https://www.desktopmetal.com/In the end it's another tool in the manufacturing repertoire. I'm not certain it will help future AAMs, but wouldn't be surprised if it did.