Multiple DataCenters is something that can be a great benefit in your business model. We're using 8 different centers at the moment, 6 of which are owned by the same company. The physical location difference is great for being able to target certain locations and keep speeds fast for users and their intended audience.
The disadvantage however is often you are often dealing with different data center companies, and as a result since you're a "small fish in a big bowl" you don't get the discounts that you'd hope for.
Keeping equipment and clients in different parts of the country/state/world means that a natural disaster in one location will not cripple your business should things go offline for a few hours or a day in an emergency.