Well to be fair, you background and service history do crop up in dialogue every so often, most prevalently in ME1, where you've also got background-exclusive side quests on the Citadel. And the service history adds extra dialogue to other side quests (for example you know Corporal Toombs personally if pick Sole Survivor.) They rarely get acknowledged in ME2, all that springs to mind is in news announcements (Shepard Memorial Plaza on Elysium for instance) and a followup email related to your ME1 background sidequest. There's more references in ME3, obvious one being Liara's holorecording of you (which also *dun-dun-dun!* acknowledges your class.) I also noticed in the archives in Citadel DLC the holo image next to Shepard is different depending on your background (it showed prefab buildings with bodies laying around for my colonist Shepard.) So yeah, lots of references to your background/service history, next to nothing for your class!