In fact, this problem looks like it's not restricted to screensaver - any Cocoa app with a text input can be made to crash. Go into Mail.app, add a new account, and type maybe a hundred characters into the password field. Then copy them all and paste them about fourty times. Hit return and boom!
Anyone who says the screensaver crasher is not really a big problem, since you shouldn't trust your screensaver for security, yadda yadda, is out to lunch. If Macs are to make it into any serious lab/business setting, you need to be able to lock the workstation when you're just going to be away for a few minutes, without saving and quitting every program.
That being said, you're probably right that the locking functionality and the screensaver functionality should be separate - any security sensitive code should be as isolated as possible from other programs, to avoid unforseen interactions (bugs in fancy OpenGL eyecandy should never get near account-protecting security measures). That's Apple's fault though, not the user's, since the only locking function Apple offers is the screensaver.