• ego, sure (how many electoral candidates have you seen who weren't on an ego trip?) ;) but more seriously:

• exposure, particularly radio/TV time these parties would never otherwise get;

• money: public financing is available even to small candidates - each candidate receiving confirmation yesterday from the Conseil Constitutionnel gets an advance of about €150,000. After the first round, if you come in with less than 5%, you can still get up to approx €800,000 in campaign expenses (the initial €150,000 included). This is set out here but it's a long legal document and you have to scroll down a bunch to get to the nitty gritty.

The second part: I'd hesitate (do I hear the tinkling glass of broken Overton Windows?) to compare Clinton or Blair to French politics. (Though Blair is often called in - almost always abusively - to disqualify either Royal, or Bayrou, or Sarkozy.) What would a Royal government be like? Well, she'd be President, and would call on a member of the PS to be Prime Minister. The government, I suspect, would be rather like Jospin's between 1997 and 2002 - which could fairly be called centre-left or social-democrat. Differences with Jospin (because Jospin, whatever he might have wished to do, was hamstrung by the forced power-sharing with Chirac who held the presidency), would be a greater emphasis on institutional change in favour of a more open democracy, and more openness to green issues like renewable energy and sustainable agriculture.

It's not insignificant that Royal represents a major party of government, while Bayrou runs a rump party that he held on to in order to mount this presidential attempt. The question one can fairly ask is, what would a Bayrou government look like? Currently, he can barely get a deputy elected to the National Assembly without UMP assistance. He floats the idea that he would gather people to him from right and left. But apart from possibly a few individuals breaking off to join him, the PS won't back him. His natural, historical family, in which he has always operated, is the right. To form a government, I'd expect him to build a new coalition with the UMP. Just how truly "centre" that would be... Well, it wouldn't... It would be the right with the UDF in the driving seat instead of the UMP, like back in Giscard's presidency.