Exactly. I can't believe people are actually upset about this. I don't see any reason to have more than one leader per Civ really. It doesn't add much to the game. I'd rather have more unique civilizations than extra leaders.
But it adds some choice

If we did not know, how the leaders for the most civs are, we would here have multiple polls about "Who should be Leader of Civ X?", and most of the mentioned leaders would be worthy to be included.
Plus it adds also a diversity in the civ. Quin Shi Huangdi represents another China than Mao. With the more uniqueness in Civ5, you could probably also design the game so, that you actually can see or feel the differences more than in Civ4.
Plus now you can't model how a civ behaves differently under different leaders.

However, due to the flavours system, the AI variety is actually going to be greatly improved.
Can't see why.
We'll have 20 factors with an maxium range of 10. In Civ4 we have 83 factors with a range of 100.