Steven Metz offers an explanation for why the administration is backing rebels in Syria as part of the war against ISIS:
Although the Obama administration would prefer Assad gone, this is not its priority. This creates tension with the Gulf states. It is also why the U.S. supports the Syrian rebels: As Middle East expert Andrew Terrill puts it, backing the rebels keeps the U.S. from being seen as simply the Shiite air force. Supporting the rebels is more about holding the fragile coalition together than directly defeating IS in battle.
Metz’s explanation makes sense, but it doesn’t say much for the value of the coalition members or the wisdom of the intervention that one of the coalition’s supposedly critical elements has been included just for appearance’s sake. This pretense might hold up for a little while, but it can’t last. As it becomes clear to everyone that support for Syrian rebels is just a temporary sop to regional Sunni governments (and the rebels’ boosters here in the U.S.), the political value of offering token support to the opposition will disappear.
In order to keep the coalition together, the U.S. would then have to provide more substantial aid, or it would have to concede that including Syrian rebels as part of the war against ISIS was always being done for show and was never going to contribute significantly to the larger campaign. The latter might cause some of the regional Sunni governments to drop out of the coalition, but then it’s not as if they were contributing all that much, either. That in turn would require the U.S. to acknowledge that it isn’t going to be able to “destroy” ISIS in Syria, which makes the decision to expand the war there even harder to justify.
It would be fair to say that almost the entire coalition has been assembled for the purpose of being able to claim broad multilateral support for what has always been a U.S. military campaign, and much of the coalition is not much more useful for fighting against ISIS than the rebels in Syria are. The U.S. touts the involvement of these “partners” so that it appears that the U.S. isn’t doing almost everything by itself, and it talks up its minimal support for local forces to make an ill-advised intervention seem somewhat less reckless than it really is. The great danger in all of this is that the U.S. could be drawn into conflict with the Syrian government if the administration feels compelled to keep up the pretense that it needs the “moderate” opposition in Syria and therefore has to come to their defense when they are attacked by the regime.