It's starting to look more and more like Congress will vote in favor of a military strike. Assad needs to be held accountable for his actions. Gassing people is not an acceptable practice and if he is allowed to get away with it with no punishment or repercussions, then the use of those weapons will continue and increase. It will also create the environment of other countries wanting to produce their own weapons to protect against those who have them and have been shown to use them with consequence. It's a domino effect and I believe it will lead to a chemical arms race in the region (the worst place in the world to have one of those) because countries will want to make sure they have more than their neighbor.
At this point, it seems the only options are for the US to do something with the good will of a good number of countries who say something must be done but are unwilling to do it or to do nothing at all. Russia has already shown they are unwilling to take any actions, either diplomatically or militarily against Syria, so we are left with two bad options. The option of doing nothing just seems like a worse option at this point given the blank check it will basically give to radical regimes all over the world who will see that they can operate with impunity as long as they get Russia or China to fight for them on the Security Council (and let's face it, who won't one of those two countries fight for?)
EDIT: It also looks like we may be getting some more evidence here soon from inside the regime (Syria Defector 'Exposes Assad Chemical Attack')