a) While this is certainly true of touch devices, it leaves something to be desired on desktop (I'll explain more below).
b) Even when you turn them back on you only get those skinny things, not the fat old ones....
Scrolling can at times be very annoying now in Mac OS X -- something that simply didn't used to be the case (no one complained about scrolling before). But with these new scrollbars the following things can happen:
1. It's very hard to navigate a long list. For example, if you are in Mail.app somewhere in the middle of your messages, swipe scrolling all the way to the top can get very annoying, very fast. On iOS this isn't a problem because you can tap the top of the screen to zip up to the top, but this doesn't exist on OS X. Sure, you can always grab the knob once it shows up, but...
2. The knob is so small now that it is actually hard to grab. So when you do need to grab it, it sometimes takes a few clicks to get it. I feel bad for anyone with bad eyesight.
3. The scrollbars often overlap the content in incredibly annoying ways, for example if you are trying to select the last item in a horizontally scrolling list, the horizontal scroller will sit right on top of it and you have to wait for it to go away or you end up clicking it instead: http://i.imgur.com/NDxTb.png . This happens all the time with the Open panel.
4. Not to mention the fact that it isn't immediately obvious anymore when you have additional content to scroll.