The French phrase for all this stuff is 'pas evident' - not that clear.
MO'L is pushing a case which will have to be sorted by the European Court (ECJ), sooner or later. The basic problem is the right of any EU citizen to work anywhere in the EU. There are rules for seamen, for example, and MO'L has been arguing that they should apply to aircrew and cabin personnel. Not unreasonable as seamen can specify how many days they were 'out of the country' and get relief of tax (and social security?) for that period.
The other element is the tax residency one - my tax adviser holds the view that the various Revenue bodies in the EU have an unwritten agreement not to poke the sleeping dog by allowing a case on this to go to the ECJ on the grounds it would last two minutes there; free movement of labour and tax residency are fundamentally at odds.
The result of all this is a typical EU mess - social security and tax are national responsibilities, but all the EU 27 have signed the Treaty of Rome which allows free movement of labour (with temporary derogations), so they need to sort the inevitable issues that will arise. In comes the economic crisis, what chance of sorting it.... none.
FR lost the French case on a ruling which applies to France, the court was not required to refer the case to the ECJ. I'd say they and other states will be very careful not to take cases which will require them to refer the question to the ECJ.
This one will run and run and MO'L will continue to prod....