I download patterns that I buy and put them in a file I created in my documents file...I call it quilt related.
It's also full of ideas and pictures of things I like.
If I really cared about it, I wouldn't let Craftsy hold it for me--I'd download it AND let Craftsy keep in my account. I have a lot of free ones in my Craftsy account, but if I actually paid for it, I go ahead and download it and make sure I have access to it.
I'd be irritated if a PDF only let me open it a certain number of times. Really irritated. In fact, if I had that happen to me, I'd probably not buy that person's pattern again and warn anyone I knew about it not to buy it. For me, it's a principle thing. If I buy a paper pattern, I can open it every day of every year and stare at it all I want--I can pin it to my wall in my sewing room and dream about it, if I want to. Why would a designer of a pdf put a restriction on you...they shouldn't, IMO.
As to what's stopping you from sharing? The same thing that's stopping you from sharing mp3s...hopefully an awareness of the law and a desire to keep your favorite designer, designing.