I can't see how increasing the pool of candidates to draw from could ever harm the fighting ability of the Armed Forces. OTOH, I can certainly see how relying on press-ganged conscripts instead of volunteer soldiers could.
That's a common misconception by people from countries who's armed forces are supposedly composed of volunteers.
If we are to believe the same people, then their pool of volunteer soldiers are usually of particular (low) socio-economic background who's last resort was the military. On the other hand conscription by its very definition includes all sectors of society, and usually weeds out the non compatible out.
In addition, conscripts both direct themselves and are directed towards their ideal goals in their military service. At least in my country the vast majority of combat soldiers request to be combat soldiers to begin with, and many have to compete on their position. Many others are hand picked for other high profile positions, from advanced technology departments, to intelligence.
In fact the debate between conscript armed forces and enlistment armed forces is not foreign to 'volunteers military', as the argument that the military draws soldiers from poor social background for whom the military is a last resort (some volunteering motivation that is) has been voiced again and again. Here is a (albeit several years old) article in The Washington Post which illustrated just that:
As sustained combat in Iraq makes it harder than ever to fill the ranks of the all-volunteer force, newly released Pentagon demographic data show that the military is leaning heavily for recruits on economically depressed, rural areas where youths' need for jobs may outweigh the risks of going to war.
Youths in Rural U.S. Are Drawn To Military
The conscripts pool is much more diverse, has a higher percentage of motivated people (people who don't enlist to the army as a last redeeming social resort), and offers much more human material for the military to use.
I think the facts speak for themselves. The Israeli armed forces is a conscripts army, its soldiers are generally considered highly motivated and well trained, and in certain departments are considered some of the best in the world. And BTW, Israeli women are drafted to the military for 2 years, with more and more combat roles being opened to them. There are Israeli women serving as fighter pilots, foot soldiers, flown medics, and other challenging roles. In general (with a few exceptions) Israeli youth are highly motivated, and already know which branch and unit they aspire to serve in with many of them mentally and physically preparing themselves for their role months and months before their draft. I believe that most of the boys that were in high school with me (and some of the girls) were taking part in combat fitness programs a year or two before our draft dates.
In March 2010 76% of Israeli youth who were eligible for combat service requested to enlist into combat units.