Number of customers is critical. When web design studios only have a few customers and one makes up a large percentage of revenue (ex: 10% or more), then you are definitely at the clients whim.

However, if an agency has a lot of customers, each with only a percent or less of total revenue, then its a much more stable platform. You can decline representation of certain clients and they do not effect the total revenue.

Of course, a product line might be better. All customers under one pricing model, customization is limited, and it scales easier. Good stuff.