User and User Account Management as a Service

Since the last few days I have been wondering as to what is the most important thing to keep in mind while developing an application for a new/startup company. And the more the think about it the more i realize the importance of having a separate application for managing user's accounts (money accounts and also the user details stuff). This is very important because you never know in what direction the company is going to grow. The company might decide to add a few more applications or a few more revenue generating business lines. Having a separate application for user account management will ensure that the business can offer all applications to all users and customers.