I have an ancient quad-900Mhz Xeon with 6GB RAM (customer does not want to migrate) that has an uptime of 1600 days, and for which total network issues during that period was a few hours (wonky power to switch).
Cloud is too often comparable to "vapor" in terms of the claims of redundancy and availability.
They also try new things with large amounts of data. This requires scaling up to additional machines for hours or days at a time, and then scaling back some or most of them when they optimize new ideas and services for production.
When the new services are a hit with clients, traffic increases and the whole cycle starts again.
Now, what I think the original post there is speaking about is for mid to large size enterprise companies that have stable, but significant traffic. In cases like this they must do a cost-benefit calculation because you risk a lot if you don't. Then the cloud might very well not be the right solution, because costs could be 10x more than anything else... so the answer in my mind is not always clear.
Sure, EC2 is probably best for a startup that expects to double every week from a nontrivial starting point and has large machine resource needs per user (viral video startups, for example). The vast, vast majority of startups won't have anything that resembles that kind of growth graph, though, and thus shouldn't blindly follow what the Pinterests of the world do. It's a completely different type of demand. If they find out that they actually are going to have double digit daily organic growth percentages, then they can switch to EC2 before it gets out of hand, but otherwise, it's premature optimization.