Hi Ken, I am not KlausC by a long shot but I can explain this for you I think.

I think the big difference here is in the wheel lug setup.

On the RWD cars, the rotor and wheel fits over a set of wheel studs that are seated in the hub and stick out away from the car. Let's call this a 'male' coupling and the rotor, wheel, and lug nut, have a female coupling. This is the 'traditional' setup.

On the AWD/FWD cars, the setup here is a bit different. On these cars the wheel and rotor are both the same (relatively, the parts aren't interchangeable from a 740 to an S70 or course), but there are no wheel studs in the hub. Instead the wheel bolts go straight through the wheel and rotor and fasten into threaded holes on the hub.

here is a link to one of these lug bolts on FCP:
http://www.fcpgroton.com/product-exec/product_id/30397/nm/Volvo_V70_Wheel_Lug_Bolt_1998_2000_/category_id/44

So you can see, if there are no lugs in place to hold the rotor up, it really doesn't have anything else there to mechanically hold it in place. In practice, between the emergency brake on the inside of the rotor and the hydraulic brake caliper (firmly bolted in place, as you noted) the rotor won't go far.

That said though, it is quite a pain to lift your road wheel up to the rotor, lug bolt in hand, and 'knock' it a bit out of alignment so now the lug holes in the rotor are no longer lined up with the lug holes in the hub behind it... you have to put the wheel down and start all over. Fastening the little pin in place keeps the rotor lined up perfectly so you don't move it while you're trying to get a wheel on there, or whatever.

BTW, always use anti-seize on the back of your rotor so it doesn't freeze to the hub, and more again on the front of the rotor so it doesn't freeze to your wheel.

I'm like KlausC and use a whole lot of silicone brake grease and silver anti-seize on everything to make sure I can get it all apart when I need to go in there again, and to keep everything moving freely in the meantime.


--
1998 V70 AWD->FWD Turbo 200k+