In the real world, tradition is well recognized as a basis for determining historical fact. The personal testimony of eyewitnesses is passed on orally until someone writes it down; sometimes it is written down very near the event in question, sometimes generations later. In our era, the oral testimony of eyewitnesses to, say, the recent tsunami are written down almost immediately, whereas in the time of the Apostles it might have not been quite as contemporaneous (though in the case of the Gospel of Luke and Acts, Saint Luke himself is a particant in events; ditto Saint John).
If you doubt the "historical accuracy" of oral testimony subsequently transcribed, then you must doubt anything anyone ever told you: Are you sure there was a Russian Revolution? Did Julius Caesar really fight the Gauls? Did a man really walk on the moon?
Judge the Gospels by accepted historical methods, and they more than pass the test for historical veracity.
Tradition can be used as a form of "evidence", yes. However, the weight placed on this evidence depends upon a variety of factors. How distant in time is the source we have from the events he describes? How objective is the source? How reliable is the source in reporting other events? Are there other, independent reports describing the same event? Etc. Josephus is a reliable source for reporting the martyrdom of James the Just because he was contemporary, he was objective, and he was intimately familiar with the events of the day. Even so, the works of Josephus as we have inherited them are not without problems, as there are a number of passages in them which are generally accepted by scholars to have been later interpolations. However, the details concernings James's death are not among them.
Other writings are not so reliable. They lack the proximity of time and direct knowledge of the events. And they cannot be considered to be objective. If you are familiar with hagiography, you are aware how quickly stories about the lives of saints could be exaggerated. I recall reading one account, written within 10 years of the death of the person in question, which attributed the most remarkable miracles to him -- which were, oddly enough, never reported at the time they supposedly happened.
Judge the Gospels by accepted historical methods, and they more than pass the test for historical veracity.
If so, you'd think more historians would agree with that.