Toshiba, Dell, HP, Acer/Gateway all use cheap plastic on their cases. They allow for better venting of the fans but you can't deny that they are not durable. Aluminum if dropped will withstand far better than their supposed "durable" plastic which Apple's entire product line now has, opposed to the other manufacturers who only use these materials on certain models. Hell even the polycarbonate macbooks withstand better. The keyboard is still better made on the macbooks with it being unibody. The screen lids and hinges are better made on the macbooks. For years Gateway overtightened the hinges on their notebooks causing the infamous screen crack. Macbooks use slot loading drives without CD trays which Dell JUST NOW adopted. I fail to see how any of these arguments have no substance.
PS. You are forgetting that quite a bit of the computing markets users are not "competent". They do click on stupid links/ads, download bogus torrents and need a more fool proof OS to save them from their ignorance.
You're wrong in that you think all pc manufacturers use plastic. Not all of their models are made out of plastic, and a lot of the times, their models that are not made out of plastic are cheaper than a comparable mac. And it's not like plastic is necessarily bad as long as it is well put together.
And let's face the facts, the #1 reason for apple using aluminum is aesthetics and #2 might because it's "more green" The only advantage aluminum offers over plastic is that it'll help protect the laptop in case of a fall better and that it possibly dissipates heat better, but it's not like the aluminum is directly covering the cpu or anything, so that's not really a big deal. Other than that, there aren't really any advantages besides looking pretty.
Also, it's not like aluminum is immune from everything. I remember the macbook pros suffering from uneven hinges, keyboard leaving marks on the display, smudge marks, swollen batteries, warping, corrosion, and the list goes on. Even the old macbooks would suffer from cracking in the casing, discolorations, etc. That's not also taking into consideration other things besides the aluminum, such as way too much thermal paste being used on the cpu, which would result in increased temps and the poor quality displays apple uses. Really, apple laptops aren't the end all when it comes to quality.
And as far as the OS, OSX is not fool-proof, it's just not many people target it because of the small market share. Whenever hackers try and take down systems at these hacker competitions, OSX is always the first to fall. It's happened the past 2 years. As OSX gains market share, eventually you'll see people targeting it, and I'm afraid osx users will be unprepared for it, so anyhow, it's always in someone's best interest to be a competent computer user, whether OSX or windows. In fact, I remember an ilife 09 torrent 2 months ago was packed with a virus, so getting one is even possible today:
http://blargkaboom.com/2009/01/27/ilife-09-iwork-09-torrented-with-virus/