In practice, finally, as well as in theory, every ray counts.
IKAROS does more than ride the sunlight; it also has thin-film solar cells to generate electricity from the light.
The nearby image was taken with a tiny camera jettisoned by IKAROS itself. Beautiful, isn't it? More here.
There's nothing in the image to provide a sense of scale, but IKAROS is twenty meters across. For the follow-on mission, JAXA envisions a fifty-meter sail.
How much pressure does sunlight exert? Very little: the measured force exerted on IKAROS is 1.12 millinewtons. By tacking toward the sun -- IKAROS is headed for Venus -- the mission will reach regions with more intense sunlight. The thing about sunlight in space ... it just keeps coming. Over time, that bit of a push will produce serious acceleration.
(A millinewton, you ask? That's the force on a Fig Newton crumb falling at standard gravity. Or maybe it's a many-legged critter eating a Fig Newton. Or Mrs. Isaac Newton. Or 0.001 kg meter squared / second squared. You decide :-)
Solar sailing is the type of project NASA might try (or retry -- AFAIK, the last NASA solar-sail attempt failed in 2008) if NASA's priority -- according to the Administrator! -- wasn't raising self-esteem in the third world.
Don't believe me about NASA's current priorities? Check this syndicated op-ed piece from the Washington Post (among many papers).
And try not to cry.