i would agree with grins here... Ich can be introuced many ways. It has stages of it's life cycle where it is free swimming, where it is latched on to fish, and even when it is in the substrate/live rock. Buying a fish from a tank with another sick one could have done it. Buying live rock from a tank with sick fish is even a possibility (although less common...). However, no method is completly fail-safe. There are flaws but QTing with hypo or copper is one of if not THE best thing we can do to prevent it from appearing in our display tanks.
I also agree that there is almost always space for a QT tank. As someone who works in a fish store, though, it is seeminly impossible to convince people this. All you need is a rubbermaid container (heck, a bucket even), something for some water movement, a heater, and some live rock or PVC if you are dosing copper or any other harmful treatment. It's not like it has to sit in the middle of the living room floor either. The backroom, a spare room, closet, basement, attic (the last two depending on the time of year and temperature), or anywhere else out of site. But there are many people who just wont do this method.
I would also not advise putting inverts in a hyposaline solution. They cannot compensate for osmotic pressures as well as fish can. 1.012 is still on the much higher side for hypo treatments, which is usually recomended to be at 1.009. But they can still DEFINATLY and easily die at that level. It is not something I would expose them to at all, nevermind long term.
And one more hypo trick for any of you starting. I have heard that ich can also have a cyst stage where it remains dormant if there are no signs of fish in the water. If you feed your tank, the oils form the food will (hopefully) trick the ich into hatching and then dying once again.
