Hi Jeff,
We work with the queue, because is more flexible and we don't believe in commodity code steering in downstream.
Above certain threshold (money, risk, complexity repetition) we try to use always contracts so no buyer is involved directly involved in the call off.
So sometimes we first start a decent tender an contract process, and ask the requester to redo his call off, but then from the contract.
Below these thresholds the aim is to process the PR as quickly as possible. It's more administrative task then real purchasing.
Btw: the non catalog node (ad hoc item) is at the front of our PR approval flow