We have explored this idea at length. The problem is, 9/10 sites for bands just don't let you share data (some examples: Virb, PureVolume, Reverbnation, Sonicbids). And they are extremely protective of the data they have.
Derek does mention the idea of customer support logging into Facebook accounts to update the content manually. That is fine for a few dozen sites, but to scale that you would need a huge staff. We're not just talking about entering NEW shows, but also being able to modify them at the last minute when data changes. That is very hard with 10,000+ bands.
The end result is, until all these sites create an open platform to share info, it just is not possible to make a cost-effective solution to make one step entry happen. I wish it was.
In the meantime, we are integrating with all the sites that DO offer open solutions (Twitter, Eventful, etc)