True, but he can also roll a high number and make the save in the 28-30 region. It makes magic a little riskier. Think of it this way - some days the sorcerer is on top of his game and casts it with power and confidence, but on other days, he rushes it, flubs it, or for some reason is not at his best when casting it. Sometimes people have good days, and sometimes bad days - and if the sorcerer is having a bad day, he could be in trouble.
To me, it adds a bit of versimilitude and takes away from the video-game feeling a lot of D20 games have (static saves, etc).