Brent Sordyl's blog

So how does loose coupling enable scaling and innovation? Scaling bottlenecks tend to move around as a site or service grows. With loose coupling it is often possible to isolate a bottleneck and prevent it from slowing down everything else. The part that’s causing the bottleneck can then be fixed (which might involve a complete re-implementation) without much or any need to coordinate with others. This also points to how loose coupling enables innovation. Different teams can more easily be in charge of their own part and make changes to it more quickly. Entirely new parts can be added more easily too (a great example here is Facebook adding chat).
(Link: One Truth About Technology Architecture: Loose Coupling)