If you look at the IMDb page of a guy like Terry Gilliam, he doesn't always work with the same people, but his movies still look like they were directed by Terry Gilliam because he tries to be in charge of every little detail. Same goes for Kubrick and to some degree Scorsese (a big part of the Scorsese style is due to the editor he always works with).
There aren't many directors like that, though. If a guy who doesn't have the talent tries to pull that off he'll just come off looking like a douchebag.
Exactly. Once again, it depends on the director and studio/producers. Tombstone was pretty much directed by Kurt Russell, but another guy got the credit as Russell did it behind the scenes after the first director was fired. John Woo was in charge 100% of his Hong Kong movies, and they kicked all kinds of ass. He gets to America, and the studios take over giving up mediocre fare such as Hard Target.
