Obviously if you haven't been watching Agents of S.H.I.E.L.D. there are spoilers in the article below. Also, (obviously) if you haven't been watching Agents of S.H.I.E.L.D., you should do so. It's fun.
Since Agent Coulson's death in The Avengers, we've seen him go through a lot of hardships. He's been kept in the dark regarding his resurrection, his beloved S.H.I.E.L.D. fell apart, appointed director by Fury but denounced by factions of agents, and he's lost a hand. He's also been kept a secret from the Avengers, even though he was the one who found the location of Loki's scepter... Did the thank you card get lost in the mail?
Many say his reveal would undermine the the Avengers because it was Coulson's death that eventually rallied them to work together -- Hence, if he is revealed to still be alive, their bond is built on a lie (and Cap will get even more pissed that S.H.I.E.L.D. is still keeping secrets). There are also rumours saying that the movie executives at Marvel don't like the TV shows because they are a burden to the films. Hence, there isn't any real urgency to have Coulson reappear in the films.
However, I believe Cap, Tony, Hawkeye, and Romanoff should be let in on the secret because of the following: first, it would enhance the show's status as part of the MCU greatly and increase its quality in terms of possible storylines/guest stars; Secondly, the films can bring back S.H.I.E.L.D. seeing as they currently don't have an organization that looks after world security; and lastly, why the hell not?
Think about it, at this point the Avengers have seen: Aliens fall from the sky via a space portal, talking robots trying to wipe out humanity, and human beings that are over 90 years old in peak condition (i.e. Bucky and Cap). The revelation that Coulson is actually still alive should not be a mind blowing event for them. They've seen/handled much stranger things.
Write down in the comments below one good reason why you think Coulson should not be revealed to the Avengers. Please don't just write "because the show sucks", I legitimately want to know what you guys think. I know Maria Hill said Tony Stark is "privatizing world security" but since the Ultron program didn't work out so great, it's time for S.H.I.E.L.D. to come back. Maybe S.H.I.E.L.D. is already present since the New Avengers facility at the end of Avengers: Age of Ultron was full of agents (they never confirmed whether they were S.H.I.E.L.D., but it looked like Fury was in charge).
I really hope season 3, or a new one-shot will have Coulson revealed to the Avengers. I don't think his reveal would work on Captain America: Civil War because the movie looks like it's already got a lot going on. If Coulson is to appear in Civil War, he would have to be revealed to the Avengers on the show first so his appearance wouldn't be out of place.