I'm afraid I don't know, because in Winnipeg we don't actually have access to that sort of service. As far as I know, it's isolated to an area in B.C. and another in Ontario, and one of those two was a recent addition. I know that the sellers who do utilize that service swear by it. They feel that it's a good value added for what it costs them to actually get to that shipping depot. They feel that they can then offer a more competitive rate to their customers, but those would be primarily their American customers. It's a tracked option for them, too, through the whole system.
From an eBay seller's point of view, the only real hindrance to that kind of approach—because it pays off in terms of the competitiveness for the American customers—is the handling time. We have an internal metric on eBay that requires us to ship within our stated handling time. If you hand that parcel off to an intermediary who's responsible for getting it into the American postal system, it can add extra handling time to your handling time, and then you potentially end up falling behind in your late shipment defects.
For the sellers to whom that option is available, I'm hard pressed to think of any who don't use it.