Yes iTunes is very good and has a lot of inventory, but still there is so much that is not still available. For example, I am a big fan of Columbo and there is nothing there - no series or individual episodes to buy or rent. Also, only 2 seasons of The Simpsons is available.
What's more, the price of an episode of The Simpsons is £1.89, which is pretty dear, given that you can buy the respective seasons for half the price on Amazon. Season 2 is £23 on iTunes compared to under £12 on Amazon on DVD (with used for as little as £3, and £5 including postage).
I seriously think each episode should be around 99p, maybe the newer ones at current price. Yes the older episodes generally tend to be much better, but still, The Simpsons are shown over and over again on various TV networks, including terrestrial so its difficult to justify charging nearly £2 each. If you were to buy each episode of season 2 individually, the total would come to around £40 or so.