"The Syrian rebels would be immeasurably weaker today without al-Qaeda in their ranks,"

 This statement would not doubt be unremarkable if it came from a supporter of Assad's view that his fight is not a civil war but against terrorists. However, the source is Ed Husain a Senior Fellow For Mideastern Studies at the Council on Foreign Relations.
    Husain claims that the main rebel force, the FSA (Free Syrian Army) battalions are divided, unorganized and often ineffective. Assad still has far superior armaments and dominance in the air and a professional army. Without outside intervention the rebels are not likely to progress very far unless the Assad regime self destructs from within. However, the Al Qaeda forces in Syria bring discipline and religious fervor to the battle. They are accustomed to fighting vastly superior forces and not at all discouraged by that. They are also often well-funded by wealthy Sunni benefactors from the Gulf oil states who want to see Sunnis regain control of Syria from the Shia Alawites. The Al Qaeda fighters in Syria who call themselves Jabhat al-Nusrah li-Ahli al-Sham (Front for the Protection of the Levantine People) are gaining both in strength and the number of their operations.
   In March there were only seven attacks attributed to the group but by June they had carried out sixty six operations including many in the key battlegrounds of Aleppo and Damascus. The group is gaining adherents from the FSA members and also foreign fighters from Libya and Iraq among other countries.
  Al Qaeda is not just interested in overthrowing Assad but in establishing an Islamist state in all or part of Syria. Failing that, they want at least to make Syria a safe haven for operations elsewhere such as in Iraq.

   As long ago as May U.S. Defense Secretary Panetta admitted Al-Qaeda's presence in Syria. However the  U.S.counter terrorism chief Daniel Benjamin said in July that the U.S. would simply ask the Free Syrian Army to reject Al Qaeda. That is probably much easier said than done. When the west supported jihadists against the Soviets in Afghanistan the eventual result was the rise of the Taliban. Something similar could very well evolve in post-Assad Syria. For more see this article.