It's important to be on Virtual Private Cloud for a few reasons. Here are a few:
1. VPC gets you out of the EC2 private IP space, and lets you configure instances without public IPs either. This helps prevent accidental misconfigurations, like accidentally letting the whole world connect to your Memcache.
2. If you want to connect a VPN to your physical infrastructure, or to connect servers across multiple regions, VPC makes that much easier.
3. Amazon's stopped launching new networking features on standard EC2, so if you want to do things like change the security group of a running instance, you can only do that in VPC.
The problem is, most startups don't think about VPC prior to launching, because these things don't matter much when you're just starting out. But unlike most early ops shortcuts, it's a huge pain to migrate to VPC later. It's basically the same effort as moving data centers, except for actually carrying servers around in your hatchback.
Now new customers won't have to think about VPC upfront, but also won't have to go through a messy and time-consuming migration. Although this won't help existing EC2 deployments, this is a huge win for new AWS customers.