That's - actually.... an AWESOME way of looking at it.
Well, it was your recent thread about the Thieves Guild that set me to scrutinizing the various factions, and asking myself how I as a player react to seeing my character faced with unsavory choices. Do I roll with them, or do I dismiss them as "out of character"? I understand that wasn't the central idea behind your topic, but that particular strand of the web was what caught me.
There is a lot of emphasis these days on "player agency"- it's a fashionable term-the ability to affect the gameworld, shape it by our own actions. We chafe when we feel forced to do things we "just know" our character wouldn't do. It's understandable. However, as much as I want my character to affect the world, I also want her to be affected by it, and sometimes that means reckoning with things that I dislike, or are simply not to her advantage. That sometimes means forgetting what
I know, as a player. When creating characters for ES titles, I try my best not to map them to the nth degree. If there is a long list of things my character would not do, or a long list of things I already know about her, then playing her becomes less like roleplay, and more like paint by numbers.
Anyway, tl;dr: It isn't pointless to join the DB if you're deeply pro-Empire. Allow your character the chance to possibly regret a few things.