A few points being consistently missed here....
First, Kurt is partially right. The clear weather is partly for the fact that they want to be able to record the test in as many ways as possible. The second part of the need for clear weather is that the target vehicle, the ICBM mock-up, needs clearer weather for launch- even retrofitted, it's pretty damned old. The weapons system, however, does not require clear weather for it to do what it is designed to do.
Second, the Boeing Airborne laser is coming online soon- but it is a Theatre weapon. Yes, it will likely be very damned effective at knocking down any ICBM it can hit during boost phase. The thing is, it needs to be actually nearby to hit the damn thing. That means that one of those airplanes flying around, say, just off the coast of California, won't be able to do anything about a missile launched from anywhere on the other side of the Pacific, no matter where it is targetted. The ICBM could fly literally directly over the Boeing during the exo-atmosphere stage, and the plane won't be able to intervene at all. This system, however, may be able to.
Third, the system may not be the most reliable. Or the system may have to deal with an extremely large number of targets. Layered defense is the best option, which means that the US wants hardened emplacements out in every friendly country it can, as well as ship-based weapon systems, because that way you DO get a lot of shots off. As for whether or not this is a good way to set up said layered defense, that is open to debate.
Which would you rather trust: One missile battery, with, say, a 90% kill ratio against a single target, (which is phenomenal given the task involved) or a hundred of those missile batteries, each with a 90% kill ratio, giving a few millionths of a percent chance that the target will make it through? So, one chance in 10 that you get nuked, or one chance in a few billion?
Yes, the system will have flaws. That's the point of multiple systems- you have one that will take out ICBMs, which go out of the atmosphere, one that takes down theatre threats, such as SCUDs, which do not leave the atmosphere, etc, etc. It's like building a house: You don't try to use a hammer for everything. You use the hammer to drive nails, you use a saw to cut wood, etc, etc.
Build a single tool to do a single job, and you can specialize that tool to be incredibly effective at it. Build a single tool to do 10 semi-related jobs, and that tool, while capable of doing a lot, isn't as good at one thing as the tool that specializes in doing that one thing.