Also in the post is that for the scenario you're describing, having dedicated servers handle the base traffic and relying on spot instances for the spikes, is much more cost effective.
Another thing to take into account is inter-datacenter network reliability. I have personally ran a hybrid system (dedicated + EC2) before and suffered a routing issue. The upstream provider for the data center that my dedicated machines were in suffered a routing outage to EC2 US-East region (or at least to the AZs that my instances were in), and my EC2 web app servers could not connect to my DB for around 10 hours (during the day time too). If you have a hybrid system, during peak hours (where you have lots of EC2 spot instances serving traffic), networking issues could result in unexpected downtime. This is of course, in addition to the regular SLA downtime you get from either the dedicated provider or AWS, which is not an issue if you have everything hosted together, but could become an unavoidable problem if not.
In terms of inter-datacenter network reliability: You have to deal with this if you want reliable hosting anyway. If you spread your database and app servers across data centers, yes, you are begging for problems and the problem is that your app is not designed for resilience.
But you can easily enough do "hybrid" within the same datacenter, if you opt for any of the number of EC2 alternatives from companies that also do dedicated hosting.
Inter-datacenter network issues are less of a problem when your datacenters are from the same provider, because they are responsible for making sure the connection is good. Plus, when problems do occur, you can troubleshoot fairly easily (when I had the routing problem with dedicated provider + EC2, I had to bounce back and forth with the network support for both a few times before one of them admitted the routing issue with the upstream network provider) as you are dealing with a single company.
I understand the hybrid solutions that other providers such as softlayer offers, but I was mostly addressing the suggestion of using dedicated + EC2 in the original article.
Only if you choose your systems so that they can't be used across both platforms. I don't see why anyone would do that if they want to run a hybrid setup.
> Inter-datacenter network issues are less of a problem when your datacenters are from the same provider
If your data centers are from the same provider, your added degree of resilience is much lower.
> because they are responsible for making sure the connection is good.
That doesn't help you when one of the data centers goes out entirely. Such as when the power needs to be cut for fire brigade safety due to a fire alarm (yes, I've experienced that), or the supposedly redundant UPS's triggers failsafes and causes the entire site to go down (experienced that too), or when one of the sites see cascading failures take out heir entire network (seen that happen too).
Assuming you will have live, working network connections between your locations, and/or that all your locations will stay online is pretty much guaranteed to cut your availability.
Basically, if your systems can't operate independently, adding an extra data center means adding more failure points.
These days there are so many hosts that offer combination of colo + dedicated servers + cloud services that you can seamlessly do both even if you don't want to deal with more than one provider.