Ideally and in de Gaulle's intent, the President should resign after his party lost an election, because it means he was disavowed by the people. But there's a reason why he didn't put it in the letter of the Constitution. The Constitution is rich with opposed virtualities, and this is why the cohabitation is not a big deal. The cohabitation has become as smooth a ride as it can be, and it is in keeping not only with the letter but the spirit of the Constitution. However, the five year term will be a big deal, but by the time anyone realizes it, it'll be too late.
vuo sez the Finnish system is also semiparliamentary. there are two things that differ in here. first of all, the president can't be a member of any political party. he is only The President of the Republic: he can't be "President, and..." presidents retire or move to international politics after their term, so they don't need to "worry about tomorrow". moreover, the idea of consensus politics is firmly rooted in the culture. thus, a strong president like Kekkonen is virtually unstoppable - his opponent has to be of the same caliper and that's not as easy as you'd think. thus, cohabitation tends to be pretty peaceful compared to that what you describe in France.
for example, now Halonen (soc.dem.) and Vanhanen (center) are "cohabitating". haven't seem much political Battleship games between them, just disagreements any two people could have, same party or no.
Sources:
- Hugues Portelli, Droit constitutionnel
- Georges Vedel, Droit constitutionnel
- Elisabeth Zoller, Alternance et cohabitation