Well, depending what sort of spinners you go against it's almost a given they'll do some damage, and I think the reason why the wheels are left exposed is so that side armour isn't there to be bent into the wheels and stop them moving. If you can make the wheels stronger, I suppose you might as well!

At least, that's what I'd assume the reasoning is. A few of the US heavies use a similar idea I recall, it's better to lose a wheel than have an entire side jammed by a bent armour plate (the two that spring to mind are Sewer Snake and Original Sin)