I have blog about how to use site. As summary, site is useful for better data security, easy governance, employing self-service and distributing administrative work. Here is some cases when sites should not be used:

• Do not create a new site if the requested site will use the same data sets as one of the existing sites, you may want to create a project within the existing site to avoid potential duplicate extracts (or live connections) running against the same source database.
• Do not create a new site if the requested site overlaps end users a lot with one existing site, you may want to create a project within the existing site to avoid duplicating user maintenance works

How to decide if a new site should be created or not? Here is my site creation questionnaire: 

1. Does your bigger team have an existing site already on self-service analytics server? If yes, you can use the existing site.
2. Who is the primary business / application contact?
3. What business process / group does this application represent? (like sales, finance, etc)?
4. Briefly describe the purpose and value of the application?
5. Do you have an IT contact for your group for this application? Who is the contact?
6. What are the data sources?
7. Are there any sensitive data to be reporting on (like federal data, customer or client data)? If yes, describe in details about the source data.
8. Are there any private data as part of source data? (like HR data, sensitive finance data)
9. Who are the audiences of the reports? How many audiences do you anticipate? Are there any partners who will access the data?
10. Does the source data have more than one enterprise data? If yes, what is the plan for data level security?
11. What are the primary data elements / measures to be reporting on (e.g. booking, revenue, customer cases, expenses, etc)
12. What will be the dimensions by which the measure will be shown (e.g. product, period, region, etc)
13. How often the source data needs to be refreshed?
14. What is anticipated volume of source data? How many quarters of data? Roughly how many rows of the data? Roughly how many columns of the data?
15. Is the data available in enterprise data warehouse?
16. How many self-service report developers for this application?
17. Do you agree with organization’s Self-Service Analytics Server Governance policy (URL ….)?
18. Do you agree with organization’s Self-Service Analytics Data Governance policy (URL ….)?