Ok, here's a couple more big reasons why it's not so useful a frame (forgetting whether it's right or not).
1. Psychology as we understand it in the West is primarily about individuals. As Robert Farr points out, American-led Social Psychology was deliberately shaped by its leaders as primarily a discipline about individual behaviour and responses NOT about group behaviour.
2. This would be ok if we were actually interested in individual behaviour, but we're not: we're interested in the behaviour of large numbers of folk and as Bernard Cova points out, aggregating up individual isolated responses misses a great deal (now Rory, just throw away the individual-channels that we can use to reach individuals - I'm talking about people and not channels).
3. Moreover, most Psych abstracts individuals from their context: other people. It puts them - we think - at the mercy of our super-fantastic-all-powerful brand marketing activity. It's a lot less messy, isn't it? But that doesn't make it right. Or accurate.
4. In the real world their interaction with each other is crucial to the behaviour of individuals. So, even the Social Psych gang tells less than half of the story because it's always trying to root things back in the individual context.
I happen to think that at the root of this lies an important cognitive bias that all of us have: we find it really much easier to respond to individual others than to the confusion of the group. It's easier to think of the face in the crowd than the crowd. Now there's a psychology insight that might be useful...
That's enough for now, eh?