And you've noted the EXACT use case for the cloud. If you are a new app, or a hot start-up and get tons of sporadic traffic, the cloud is absolutely where you need to be. To do anything else is beyond foolish.
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.
I believe friendfeed ran off of a single machine for most of its life. A few years down the line, I think most people who have only used EC2/Heroku would be shocked at how much traffic a single recent Xeon 8+ core machine with 32+ gigs of RAM and RAIDed SSDs can handle and the price at which it does it. Before that, even a mid-tier VPS is probably a better option than EC2 for most.
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.
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.