If the firmware loaded to the drive by some OS corrupted data or parked the heads frequently, it wouldn't be all that different from the OS corrupting data or parking the heads itself, particularly since the firmware would only be present while that particular OS was running. I'd much rather trust that my OS implemented the filesystem right twice than trust that both my OS and my hard drive manufacturer implemented the filesystem right. Of course, it would only run the OS's firmware in the drive processor's userspace, and the drive's kernel (and ASIC portions) would take care of the important stuff.