Lawrence "If Kuwait grew carrots, we wouldn't give a damn" Korb, an excellent commentator on military affairs, lays a smackdown to advocates of space-based weapons. Essentially, the oldest and simplest argument against space-based weapons is still the best. In essence, for every dollar the US spends on space-based weaponry, it's adversaries will spend pennies on countermeasures - and the adversaries will win out. This is especially true for things like space-based Ballistic Missile Defense, but even for the less fantastical ideas. Any weapon placed in orbit is immediately seen, and is incredibly vulnerable. As many people have pointed out, once you've put a multi-billion dollar satellite in orbit, it can be destroyed by a million dollar rocket with a $10 bucket of nails.
This is why, back in the day, the US agreed with the USSR to cease work on anti-satellite weaponry - it was destabilizing to the entire surveillance satellite network of both countries. The only recourse was for each country to threaten nuclear war if their satellite were blown up, which wasn't somewhere even Reagan wanted to go. Bizarrely, space-based weapons that were originally hoped to escape Mutual Assured Destruction ended up only strengthening it.
Go read the Korb piece. He's generally worth reading.