That didn't happen in this debate, that was spin. As someone who has done a tour in Afghanistan (in the horn of Panjwai, which is arguably more dangerous than most places in RC-E they were talking about), I can tell you that our soldiers are already out and conducting patrols in all those places.
Both men were bending the facts. Biden was saying it was okay to reduce our troop levels ahead of the surge because Afghans are fulfilling those roles on a 1-to-1 replacement ratio. That's entirely untrue, and even if it was mathematically correct (and it isn't), the Afghan forces I worked with over there are not on the same level as our soldiers and marines.
Ryan was distorting the timeline of events. General Petraeus and Admiral Mullen were saying it was a bad idea to draw down our total numbers from our surge prior to the fighting season. Traditionally, that fighting season runs from ~March-August, give or take a month either way. The surge troops remained in theater for the bulk of this past offensive. Ryan was trying to make the case that they wouldn't be there for the NEXT spring offensive, but that was never part of the deal. By that type of faulty logic, we'd need to leave the extra troops there indefinitely.
Poor arguments made by both men. Both were wrong on that one.