I understand orginally wanting him to make Indy an Anit-Hero (more like in Temple where he's all 'fortune and glory'), but I think making Marion 11 would've just completely destroyed us wanting to root for Indy.
As an audience we can forgive our heroes for a lot; being alcoholics, dead-beat-dads, high priced assassins who kill many people, drug addicts, etc. But I think the audience would've collectively turned away from Indy being in a realtionship with an 11 year old girl. Maybe I'm wrong, but I really don't think the audience would've been on Indy's side after that.
So, more's the better that they ditched that idea and upped the age.