The design itself shows off imperfections more than anything else, and the shoehorning process into the thin dimensions has meant a weaker chassis.
I think that despite protestations otherwise, people do baby the MBP - it's to do with the design once again. because it shows dirt, marks, etc and it's an investment for many, they do tend to baby it as a matter of course. A Dell wouldn't get that kind of respect.
I think it's OK if you don't cart it around that much - my 'unbent' 17" is still managing to stay straight with a portage once every two weeks or so, and the 15" seems to be holding up OK although I did send this or the other one back due to an excessively bent screen. That I have to worry about something like this is pretty unique to me since I've switched from (and now back to) machines designed properly for the purpose they're to be put to.
The aluminium is scratch resistant - it's hard-anodised, and the anodising layer seems to be reasonably thick. But once you've scratched it or worn off the anodised layer, that's it - you're left with something that's basically weaker than even ABS. All in all I'd say it's OK - but it's not durable in the sense of other more travelworthy laptops.