Yes, more troops are necessary to deprive Al Qaeda of a safe haven in Afghanistan, because Afghanistan has harbored terrorists through a long period of time. Making sure that this kind of environment does not rebuild is critical to fighting terrorism. The United States has invested a great deal in the war in Afghanistan and should finish the job.
Sending more troops, or keeping them there at all, simply kills civilians. We have had our army there for over a decade, yet there has been little change in Afghanistan. Also, there is absolutely nothing left in Afghanistan or any nearby areas that could pose a threat to the American people; the war is basically useless for that reason and sending more troops there will make both Americans and Afghans suffer. There is little advantage to sending more soldiers, but a very high price, and one we shouldn't pay.
The War in Afghanistan is not necessary to deprive Al-Qaeda of a safe haven. Pakistan's tribal regions have been offering a safe haven for decades as well. Strong national governments in the region are needed in order to stop terror cells and terror camps from existing in the first place. Only when governments refuse to negotiate with terrorists, and only after this region becomes more prosperous in the world, will terrorists be unable to find a safe haven anywhere on this planet.