Is a Data Warehouse really necessary?

The data needed to provide reports, dashboards,analytic applications and ad hoc queries all exists
within the set of  applications that support your organization. Rather to use all those applications separately Why not just use one of the Business Intelligence  tools to obtain it directly? 

BI pioneers have discovered the hard way that the “direct access” approach simply does not work very well. There is no reason to add the name of your organization to the long list of failures. Some of the many reasons why direct connection almost never works include:

New releases of application software frequently introduce changes that make it necessary to rewrite and test reports.

• These changes make it difficult to create and maintain reports that summarize data originating within more than one release.
• Field names are often hard to decipher. Some are just meaningless strings of characters.
• Application data is often stored in odd formats such as Century Julian dates and numbers
• without decimal points.
• Tables are structured to optimize data entry and validation performance, making them hard to use for retrieval and analysis. 
• There is no good way to incorporate worthwhile data from other sources into the database of a
• particular application.
• Developing and storing metadata is an awkward process without a data warehouse – there is no  obvious place to put it.
• Many data fields that users are accustomed to seeing on display screens are not present within the database, such as rolled-up general ledger balances.
• Priority always needs to be given to transaction processing. Reporting and analysis functions tend to perform poorly when run on the hardware that handles transactions.
• There is a risk that BI users might misuse or corrupt the transaction data.
• There are many ways in which BI users can inadvertently slow the performance of applications.