I don't know that Apple needs to position the MBA vs the base model MBP as anything other than cheaper.
Over time I've come to realize that the lower-end MBPs aren't necessarily purchased for their capabilities, many times they're purchased because someone who sends emails and pushes online spreadsheets back and forth is a professional and "needs" (demands) a "pro" laptop. So IT goes and buys them a "pro" laptop. They happily set it on their desk where it will live 90% of the time, plug their old monitor into it, and are happily "pro"-ing away. Even though the Air could do precisely the same job just as well at a reduced price.
In my opinion the brand and naming scheme sell those base model MBPs more than anything else.