People say the YIJC show an Indy that acts "out of character". To that I could counter that in LC, Indy acts ''out of character'' with ToD. He goes from being a treasure hunter in search of nothing more than ''fortune and glory'' to becoming a conservationalist who believes that the artifacts he discovers ''belong in a museum'' in the space of 3 years.
But Indiana Jones is a realistic character in that his personality, and his views on things, changed over time, like a real person's views and ideas would.
What we see in YIJC is his life. Henry Jones, Jr. didn't come out of the womb with a fedora on his head and a whip in his hand. The alter-ego, ''Indiana'', formed slowly, starting with an innocent rebellion in Indy's rejection of his true name for that of the name Indiana.
The events shown in the YIJC (or Adventures of Young Indiana Jones) help to mold Indy into what the man we see in the movies.
In WWI, Indy learns how to use deception and how to improvise on the spot, something which he would come to do quite often in the movies. We see him having a bit more experience with the whip--a foreshadowing of how important a tool it would become to him--We see his first fist fights, going from a naive fighter to a dirty fighter by the end of the series. We see his first brushes with corruption and dissapointment, which would begin to bring his cynicism to the surface. We see his first true treasure hunt. We see the distance between him and his father and the gradual breakdown of their relationship. We see how Indy became interested in archeology. We also see how he learned all the languages he spoke in the trilogy (and many more) and how he became so knowledgable about the ancient world.
Between 1916 and 1920, Indy dates and has casual flings with a number of women (years of field work), and by the end of the series, has no qualms about dating three women at once.
People say it's unrealistic for Indy to meet all of the famous people he meets--but in the majority of cases they weren't famous when he met them--and isn't it even more unrealistic for Nazis' faces to melt? For a man to age and turn to dust within seconds? For a 700 year old knight to still live? For a man to live without his heart in his chest? Is it not impossible (or at least very very difficult) to actually perform the truck stunt in ROTLA the way it looks in the movie?
So I would say when we're watching the YIJC, we are truly watching the early life of the man we see in ROTLA, TOD and LC; the historical aspect of the show is simply a bonus.