If I may jump in here...

t:
The FF need to be in the Marvel Cinema Universe. Their presence makes a universe starting to run out of two dimensional characters even richer. In the 616 they are a key role. An outside force other than the Avengers that are probably more popular and more feared throughout the Multiverse. They are the ones the Avengers call when things get really effed up. Funny thing is... it's not even their job. They are of course a family of explorers. But are lynchpins to how the 616 works.
Before Kang was Kang he was Rama Tut.
Since Ben and Sue have been so eloquently defended by Pink Ranger, I won't go into detail, but I will say that Ben's heart of gold and never quit attitude makes him one of the most loved characters in all of comics. And Sue has had the most growth of any female character in comics. Her connection to Namor also sweetens the pot.
As for the other two. Reed? The smartest man in the universe? No room for Reed in the room? One of the best things about Heroes Reborn was the Atomic Knights of the Round Table.
Reed
Stark
Banner
Doom
And then some spare that should have been Pym. Anywho... those characters ARE the Marvel Universe. They bounce off of each other well, as displayed in the Avengers with Banner and Stark. Now Banner might not have the gumption or desire to one up Tony to his face, but Reed has no such qualms. I'd love to see those guys together.
Johnny is Johnny. Not everyone is destined to save the world. But he has. And dammit he's fun. Put him in and watch him react to tragedy. It's always heart wrenching.
Do I need to talk about Doom and Galactus and the Surfer and the Negative Zone and...
The List goes on.
There's plenty of room for the FF. As a matter of fact, the Avengers will absolutely need a punch in the arm at some point. How do you top bringing all the solo heroes together in one movie?