I think we're all hitting relevant aspects of the issue here. One of the things I really like about Arduino is that you seldom feel like you're 'hitting the metal'. The Arduino language feels like a real programming language, while the others I've looked at (MSP430 and PICAXE) are more like strings of configuration parameters, hiding the logic of the program.
I'd far prefer to be able to understand programming logic concepts than remember specifics of particular processors. For more than half my life, I've been uselessly carrying around the ability to disassemble Z80 opcodes in my head, even though the need for me to do so evaporated more than 20 years ago.
I program Arduino for fun. My day job is engineering, so I leave the detail stuff in the office. I'm glad other people enjoy squeezing the last clock cycle out of their processors. But there is, as the old Perl mantra goes, more than one way to do it.