I guess I have a 18th order desire. This desire is to have a 17th order desire. Why would I want to have a 17th order desire? Because it would be so cool to have a desire of such a high order. And I could pull out my 17th order desire at parties with other philosophers and impress them. So I've got both an instrumental and a non-instrumental reason for my desire. And I wouldn't be surprised if this desire turns out to be settled.
I am not quite so desirous of having a 16th order desire. Sure, it would be kind of cool to have a desire of such high order, but I think prime number order desires are cooler.
What's the point of this little tale? Simply that higher order desires, no matter of how high an order, can be just as frivolous--and probably even more frivolous--than the typical first order desire. There is, thus, little reason to privilege higher order desires over lower order ones, giving them some kind of an authority whereby they get to define our welfare.
Maybe you'll question whether anybody can really have that 18th order desire. Well, I'd like to have that desire, and maybe I actually do, if only so I could honestly brag about it (that's not actually an instrumental reason of the crassest sort, because of the "honestly"). And that means that I I've already got a 19th order desire, namely the desire to have the 18th order desire mentioned at the beginning of this post. I wouldn't be surprised if this 19th order desire lasted for quite a while.
And, hey, it would be quite cool to have nth order desires for every prime number n, and to have no non-prime order desires except for the first order (I guess we need to have the first order ones to make sure we don't forget to take care of our bodily needs). Suppose I actually want that, and that desire is settled, reflected upon, etc. Then here we have an infinitieth order desire--and as frivolous and unimportant as desires get.
Or maybe you'll object that these very high order desires are very weak. Sure. And it would be a mark of insanity if they were very strong. But that underscores my point that there is nothing deeply rationally special about high order desires.