Union and Davis are incorporated into the fabric of Camberville (god I hate that, but it's an efficient portmanteau) in ways that Assembly just isn't, and the businesses and development pattern are much more centered on the neighborhood than Assembly, which is designed as a destination with a neighborhood, rather than a neighborhood that is also a destination. Obviously Assembly has a growing population, but parsing "downtown" vs "outpost" vs something in-between is pedantic. Inasmuch as Assembly is incorporated into its surroundings, it's definitely an outpost in my opinion.
Regardless, we all seem to agree that Assembly both needs and wants drivers to visit the area, and that it's highly unlikely that the existing garages will ever be converted to other uses.
I went to Assembly at lunch today with my girlfriend to run a few errands (missed the new OL train by 7 minutes, but that's a different story). It's actually more convenient to get to from our offices downtown (both pretty close to the orange line) than by any transit option (even driving) from our place in Somervillle. We were talking about why we liked living where we do vs. renting she pretty much echoed what you're saying here. She said, "I like that we're close to Davis, but it's an easy walk to the smaller squares like Teele, Ball, Magoun and Porter which all have a different feel and are a great change of pace close by." And that's it. I don't even take issue with Assembly's supposed lack of authenticity" or anything along those lines. It's the lack of integration into the fabric of the city/area that's my biggest gripe. Hopefully that changes over time, but it's still an excellent development and an asset to the region.