The real problem with keeping Europeans on the line in Afghanistan (and earlier in Iraq where they largely did not fight even when they deployed--except for the British) stems from lack of capability and the fact that Europeans to a large extent side with our enemies.
On capabilities, Gates slams the Europeans for failing to maintain armed forces capable of contributing to collective defense.
And on the sympathies angle, Strategypage notes:
The enemy is encouraged by the recent Dutch decision to withdraw its 2,000 troops from Afghanistan. The Taliban deliberately targeted the Dutch because they knew that the leftist parties in Holland were eager to get out of Afghanistan. The leftist parties in the West are more likely to be sympathetic with Islamic radicals and prefer a policy of no military intervention. The Islamic radicals play on this, with some success, at every opportunity. Australian, Canadian, German and British leftists are now pressuring their governments to withdraw, the implication being that the Americans will take care of it or, if the Americans pull out, it won't really be a problem that military forces can do anything for.
Given the general sympathies of so many European leftists, maybe it is a blessing in disguise that the Europeans lack the capabilities to fight for what they believe in.
UPDATE: Strategypage covers the casualty-rate issue in Afghanistan.