Google's Chrome OS Means Losing Control of Data, Warns Richard Stallman

Yes, he has a very good point. No matter how stable, redundant and distributed the cloud backend is technically, the danger is in the human factors.

Especially if you depend on one company (either Google, Amazon, ...). If that company decides you violated their terms of service in any way, they might simply remove your account and all your files. There is nothing you can do against that.

This would make it even worse than 'traditional' vendor lock-in with proprietary data formats.

Luckily, most cloud service providers allow downloading your files to make remote backups. But this might not always be the case and is easy to overlook/forget.

Choosing to let some company hold on to your data isn't exactly giving up freedom. You still have the freedom to not store it with them and you haven't given it up, but merely opted not to exercise it.