Short answer - catch your shin on your opponent's knee or elbow in a roundhouse kick without shin guards and see how you feel. ;)
Or, to look at it another way - Do targets defeat the purpose in training? Does a face mask or mouth guard defeat the purpose of training? Does practising with dull/not metal throwing stars or a wooden blade defeat the purpose?
These are all tools designed to keep you safe while learning the techniques, and they have their place. As stslavik mentioned, randori is essential, but it's still not the same as getting in a real street fight, making it, by nature "not realistic." Randori is a lot more controlled, and your opponent has training, and you're both sober. In a street fight, your opponent is most likely not trained, may be drunk, may have backup, and the situation as a whole is a lot less controlled.
I tend to liken Ninjutsu with chess - when it comes down to it, the moves themselves don't matter much. What matters is that you're thinking two or three (or four) moves ahead of your opponent. Knowing the moves gives you the ability to determine what to do in a given situation, and power to plan them ahead of time, but when it's all said and done, what matters is what worked.
As David said, if you want to toughen your shins up, do conditioning exercises that will do it gradually and safely. You'll do more harm than good to yourself in the long run if you're constantly out for weeks at a time for bone fractures.