AeroMexico gives free seat selection (any seat in the whole Economy cabin) to any SkyTeam Elite, i.e. also to Silvers (like me). On a AMS-MEX-ACA-MEX-AMS itinerary I got about EUR 300 worth of free seat upgrades (premium economy on the long leg and front of the plane with extended legroom on the short leg). You can't select the seats online but if you call or chat with them they are very happy to help.