Cap is not needed... Reed or Cyclops are better candidates for leaders... have Surfer instead of Gladiator...
Cap is a very core part of the Marvel Universe though, less so than Cyclops, and the FF are contained in their own niche. Cap is also an early creation of Jack Kirby, and a Golden Age character who's survived the best through the years for Marvel's pantheon. He fully deserves his spot. I may not like him all the time, but I have to give him respect.
If I had to pick 12 characters to make up Marvel's JLA, I'd go with a mix of powers, position and popularity.
Captain America
Namor
Human Torch I
Hank Pym
Wasp
Hulk
Thor
Iron Man
Spider-Man
Doc Strange/Captain Marvel--I get the feeling that Marvel very much wants Genis to succeed commercially, just they can't figure out how to properly market things.
Wolverine
Daredevil/Black Panther--BP for the same reason as Captain Marvel.
At first we have the Golden Age triumverate, then the founding members of the Avengers, who were all big sells back in the day, as was Doc Strange. Wolverine is arguably the most popular mutant, though I feel some of the originals should be on the list, but for so few numbers, they just don't cut the mustard. Spidey and DD have both held a steady monthy title, at least, throughout the years, and are staple Marvel characters, as I feel Marvel wants BP to be as well. Wish they had a few more well developed female characters though. I think She-Hulk's come into her own as a strong central character, but we need some more.