I understand what you are saying, but other UNSC members with a veto can only stop the UN from taking action, they can't stop us from doing what needs doing outside the UN; e.g. Iraq and Kosovo (horrors! a Dem administration acting without UN consent! Doesn't seem to get brought up too often when Bush is castigated for doing the same!)
Let's say we have to go into Iran because there is firsthand intel that shows the mullahs to be just months from getting the bomb and we go to the UNSC for a vote and China and Russia veto. But our leaders, hopefully, decide that our national security is too important and attack/invade Iran anyhow to neutralize the threat. If we act contrary to the UNSC permanent veto holding members then who is to say they won't do the same sometime in the future when we try to use our veto?