Large mining pools appeared not because ASICs became widespread, it happened long before. It's not ASICs what makes people grouping in pools, it's that their share of total hashpower is too small.
Imagine a million of miners having equal hashpower mining Bitcoin (doesn't matter they use CPUs, GPUs, ASICs or some 'alienware'), it's still only ~144 blocks which they mine together in 24 hours. Individual miner still has only about 5% chance of finding a block in a year. That's why miners unite in pools. And they started doing so long before ASICs.
Today mining is more decentralised than it was before ASICs were introduced, when some pools (like Deepbit, BTCGuild) were dominating (each at its time), having around 50% and even more of total hashpower.