It's all very well being 'in NATO' - but if they don't offer up their bases or a firm commitment of forces then saying you are 'in NATO' is next to useless.
Imagine if you are trying to plan convoy escorts - would you risk thousands of lives on whether French escort ships are going to turn up / be allowed to fight?
NATO as the name suggests is an alliance - it's not for dipping in and out of as you see fit, especially as the French did during the Cold War.
From Wiki:
http://en.wikipedia.org/wiki/Nato
".... de Gaulle continued his pursuit of an independent defence by removing France's Atlantic and Channel fleets from NATO command. In 1966, all French armed forces were removed from NATO's integrated military command, and all non-French NATO troops were asked to leave France. This withdrawal forced the relocation of the Supreme Headquarters Allied Powers Europe (SHAPE) from Paris to Casteau, north of Mons, Belgium, by 16 October 1967. France remained a member of the alliance, and committed to the defence of Europe from possible Communist attack with its own forces stationed in the Federal Republic of Germany throughout this period. France rejoined NATO's Military Committee in 1995, and has since intensified working relations with the military structure. France has not, however, rejoined the integrated military command and no non-French NATO troops are allowed to be based on its soil. The policies of current French President Nicolas Sarkozy appear to be aimed at eventual re-integration."