For many of us, if we didn't need AWD we wouldn't bother with Subaru. For me, this car with a hand-built AMG engine (4 cyl or not) and AWD is enticing.. and it seems like fuel economy could be a bonus. I don't refute your point, but we all cross shop different.
Also, comparing a mustang to a merc is really quite silly. Then again, comparing a merc to an STI is silly as well. My 2011 STI was seemingly built by either new, blind, or mentally challenged technicians.