Since Javik is a Prothean who has already dealt with the reapers and understandings things a bit better, if you complete Mass Effect 3 with him as a squad member do you gain more insight about the history of the Reapers?
You learn some information about the Prothean war with the Reapers, but not very much. His comments about the topic mostly focus on how his race got wiped out. With basically no more information about the Reapers themselves.
He does talk a lot about the way Prothean society operated, how the empire spread, their views on race, culture, warfare etc.
Mostly you learn just how wrong the Asari are about Prothean society. Liara is not impressed.
I second him being a racist jerk.
You'll gain some insight, but it's not really into the Reapers.
You'll learn a bit more about the Protheans, as well as the early days of the younger races. For instance, Javik is fond of reminding you that Salarians used to eat flies.
He's... kind of a racist jerk.