I do believe(trying to recall that World Civ class I failed years ago) that the whole point was the rest of the Greeks left to go do something else relating to the war, while the Spartans volunteered to keep the enemy busy and buy everyone else time. (That's the "honor and glory and we're going to save Greece with our sacrifice" deal. They don't just get slaughtered for no reason.) The
Wikipedia entry seems to agree, more or less, except that there were also a bunch of Thespians there too. In the graphic novel, it's mentioned that other soldiers do decide to stay on with them, and it's implied at one point in the movie, but the point is that the Spartans were like kill-bots who fought bravely and expertly until they were overwhelmed by sheer numbers.
300 is visually awesome. The music's a little hit and miss--when it's suitable, it's great, but there are a few times where it just feels completely wrong--and the acting's good enough to do the movie justice. Mostly, though, this movie relies on its visuals, which not a problem as it is freaking awesome just to watch. I knew it was shot in front of a bluescreen, and I watched the backgrounds at first for that reason, but I wouldn't have been able to tell the backgrounds were added if I hadn't been looking. The plot and dialogue are great too, but I wish it had been bloodier.
"Oh, better far to live and die/Under the brave black flag I fly/Than play a sanctimonious part/With a pirate head and a pirate heart."