Then there's people like my aunt. VERY partisan. Can discuss and debate issues all day long, and usually has her basic facts straight, but... she's still a partisan hack and goes into every candidate and issue with a pre-conceived bias. She votes straight party line as well.
I know others who vote straight party line, but not because they are party-blind. It just ends up working out that way more often than not. They do put honest thought into the process, but the alternatives are too different from their philosophies and aren't appealing to them.