I remember one tire company advertising that "your tires are the only safety feature on the road - that actually touches the road". While I do not intent to go on about any specific brand of tire, I would like to make the point that your tires (and wheels) are perhaps the most important safety item on your bus. Adapters, while alright assuming they are designed for the loads they will see, still add another link between "your bus" and the road.
Consider this. You're going down the road at highway speed and someone cuts you off, or a deer darts out in front of you, or any other factor which makes you need to make an emergency maneuver. That's going to be when it puts maximum stress on the adapters (along with everything else, if you think about it), do you *really* want the possibility for failure to be there? Wheels, lugs, tires, and everything else *could* fail, but the likelihood is exceedingly low; they are designed for the load they are carrying and you *did* check everything before you left ... *RIGHT???*