If you extrapolate that suggestion, no agents would ever read any fulls. They'd all be too busy waiting for someone to offer before they started.
I don't think agents purposely wait until there's an offer to read the fulls they request. It's just that they're so busy that sometimes they don't get started on those fulls for a while, and when they hear there's an offer they have to race to catch up.
I would like to think that agents are all hungry to make deals, but time and again, I’ve heard from agents as well as writers about agents who...aren’t. Because of the way publishing has operated, at least in NYC, with all the unpaid internships etc, the only people who can afford to be agents in NYC are, well, wealthy ones. I know of at least two agents, one from personal experience, who have barely made any sales in the past five to ten years, and yet they can still afford to live in NYC.
So I’m guessing just as with any other industry, publishing will have its hard workers as well as its duds. Not every agent is hungry. Not every agent is passionate about agenting. Some treat agenting almost as a hobby. And yeah, with the ones who can afford to lay back and take it easy, I can see them waiting for the MS to get an offer before reading. If they lose the MS, meh, they’re not going to starve. I think they’re in the minority, but they’re there.