Quinto/Tumba Pairing??

I've noticed on Youtube a couple fine congueros and Tomas Cruz's in parts of his instruction DVD's playing a quinto and tumba pair without a conga. What little I've heard sounds good; however, I've never really seen anything written about this other than "preference". So I wonder what shaped this preference? What are these players listening for? Are there setup guidelines for the practice? Are there genres that this works with better than with others? Is it practical for one wanting to play gigs? How common is it period? What do you think about this pairing?