My guess: It seems clear that Satoshi didn't expect pooled mining.
In a world without pooled mining, you'd simply have each piece of mining hardware capable of up to 4 gigahashes per second (GH/s) use its own public key, guaranteeing that it produced a unique coinbase transaction output. The time field can be updated every second, so the nonce can be reset to 0 whenever time is updated.
In a world with pooled mining, multiple people are all creating identical coinbase transaction outputs (paying whoever the pool operator says to pay) and they're collectively hashing at much faster than 4 GH/s, exhausting the nonce range before the time is supposed to be updated. This makes the extra nonce required to avoid having multiple miners check the same header hashes.
So why did Satoshi create the extra nonce in the first place? It looks like it was an easy way to see how many hashes the miner has used since being started. If you look at the coinbase from block 1 (the first block after the genesis block), you see it is:
04 ......... Push 4 bytes to stack
ffff011d ... The same as the nBits field
01 ......... Push 1 byte to the stack
04 ......... Number of times nonce was reset so far: 4 <= 20 GH
By block 10 it's 0x36 (54 <= 220 GH). In short, the original extra nonce may just be an extra debugging tool. You can use nBits and the extra nonce to calculate how many blocks on average the miner should've produced, and if that's very different from how many blocks it did produce, you might have a problem.