There are three main advantages to using ExcelWriter instead of Microsoft Excel as your server-side Web reporting application.
First, ExcelWriter opens and creates spreadsheets on the server faster and more efficiently than Microsoft Excel. Using ExcelWriter with more than just a few users shows a significant performance gain over Microsoft Excel.
Second, ExcelWriter is a scalable application that can handle creating spreadsheets for just a few users or for thousands of users, while Microsoft Excel only works well for a small number of users. The difference in scalability is easily noticed when multiple users want to view the same spreadsheet simultaneously. And not having Microsoft Excel on your server can spare valuable server resources, which are normally lost when multiple users access a spreadsheet at the same time.
According to Microsoft, Office Applications such as Excel, were never intended for use server-side and by using Excel you are taking risks with the stability of your overall solution. See "Considerations for Server-Side Automation of Office (Q257757)" at http://support.microsoft.com/default.aspx?scid=kb;en-us;Q257757.
Finally, if you are going to run Microsoft Office on the server, all of the clients that access Excel spreadsheets from the server will require a Microsoft Office license. Again, according to Microsoft, it is impossible to run the Office Web Components (OWC) on a Web server that is accessible to users on the Internet or extranet because there is no way to guarantee that all users accessing the OWCs have valid licenses. See "Licensing the Office 2000 Web Components and Office Server Extensions" at http://support.microsoft.com/default.aspx?scid=kb;en-us;q243006.
On the other hand, using ExcelWriter as your server-side application does not require a Microsoft Excel license on the client. To view and modify spreadsheets you can use free programs like the free Microsoft Excel Viewer or Star Office.