Like I said in regards to your quotes on the constitutional phrases, the Supreme Court refused to actually rule on them is what I'm saying. Nowadays, court and historical precedent is often times perceived as more important - you can look at the stretching of the interstate commerce clause by courts and legal precedent as an example. If a Governor said "no", the Governor's decision would stand a very good chance of being upheld simply due to precedent - regardless of what's in the Constitution - which applies to most things these days, which is sad.
In regards to combat effectiveness, they actually highly vary. Most are used just as a disaster-relief force, though States like Texas, Tennessee, and South Carolina have 'combat' components that get the same type of training National Guardsmen do (weapons training, and the "one weekend a month, two weeks a year" drilling).
Also I'm pretty sure Thomas specifically stated he was talking about State militias.