Erod,
The rationale for a timeline was given, I think, for three reasons:
1. to ease the minds of Americans, our military, our allies and Congress that the military objective should be at its conclusion within a "reasonable" period of time. The President was correct in assessing that people domestically and abroad (our allies in this fight) are nervous about committing more troops and money to this cause. Therefore, the President, as the allied NATO leader in this War on Terror, had to reassure America if not the world that he plans to put an end to this war effort in the foreseeable future.
2. NATO and our allies needed to know that although we are committed to this fight, we have not intenstion of being there forever. The shorter timeline w/resolved behind it leave room for those nations who have been sitting on the fence to reconsider their level of involvement or non-involvement. Some will likely say, "Since we haven't been involved in this matter we'll continue to stay out of it and watch from the sidelines," but others may say "Since you're staying the course but moving in a different, more define direction and you've provided a viable timeline for withdrawl, we'll commit to (____, be it financial aid or more troops) and help see you through this to its conclusion".
3. To reassure the Afghan people that we're not occupiers and will leave their country as long as they step up and play a hand in helping us win the fight.
All three were critical!!! And thus, the reason issuing a timeline was so very important. Unfortunately, so many people have gotten so hung up on this one aspect they don't stop to think of the wider ranging implications. Thus, the narrow point of view tends to be, "why give the enemy a reason to sit back, wait us out and regroup?" Well, if your military does their job right, there shouldn't be a remnant of Al Quaida or radical Taliban remaining.