Not enough? Perhaps worst of all, after putting his wife and child through never having been around, hitting Cynthia in his sporadic rages, and having hundreds of affairs, he decided that he'd prefer a different family with Yoko Ono and deserted them. At least Cynthia Powell Lennon was an adult, someone who had learned how to be responsible and fend for herself... but Julian? He was five years old when John Lennon abandoned (and that's the word for it, plain and simple) him, and over the next twelve years apparently saw his father on something like ten occasions. Julian was able a few years ago to tell a reporter who asked what his dad was like, "Well, you probably knew him as well as I did." Does he seem a little unforgiving? There's a reason: his father's behavior was unforgivable.
If I were describing anyone but John Lennon, you'd read those last two paragraphs and say, "Wow. This guy was a real dick."
Humans have a need to idolize each other, and idealize each other—that is, we need not only to admire people for what they do and did, we need to project our admiration for them onto the things they didn't do. It makes us feel hopeful about the human race to think that people are capable of greatness in many facets of their lives. It's not a bad thing. But it becomes a bad thing when we take someone who was SEVERELY lacking in certain of those facets and pretend that he/she was a model of human behavior.
Because when you do that to a person, another person will find and reveal his weaknesses, and the revelation ultimately damages not just the idol's reputation, but the morale and ideals of the people who found in the idol false reasons to believe in humanity. Sound cheesy? It is, but it's also true. Think about how you'd feel if you found out tomorrow that Mister Rogers was a convicted sex offender: you'd be devastated. I would be. (He wasn't, just to clear the air of that.)