a divide between the UI and everything else

I've worked two places so far where there has been a predisposition to, instead of setting up onion architecture for new projects, keep core logic and infrastructure mechanics in the same Visual Studio project. In effect the solutions have "two layers" wherein one is the User Interface and the other is this other animal which the UI inherits. We call it the "Data Access Layer" where I work currently. Core logic and infrastructure mechanics are still split apart by the usual interfaces and StructureMap approach. My current job is, again, one of two where I've seen this and, moreover, when I was at a third place of employment, I witnessed a lively debate between two coworkers over whether or not this was the way to go or if, instead, the onion architecture was unquestionably sound. The antagonist for the onion architecture suggested that the deployable code beyond the UI should really be something that one might bundle into one .dll in the name of clean packaging. Interesting stuff.