More strikes indicates the union is less powerful - a powerful union never has to strike, because they always get what they want just through fear of a strike.
Unless the threat is exercised every once in a while, it is not taken seriously.
German and French companies know that unions are capable of and willing to go on strike if their demands are not met. That gives companies a much larger incentive to make concessions.
The Warnstreiks I mentioned earlier are a tool for reminding companies every once in a while that the possibility of a strike exists. They're often used during contract negotiations, when the union doesn't feel the company is giving enough.
