I don't understand the BI, we know that excel is THE solution used by the business users to do their job. I have difficulties to see mashups helping in any way?!?

Spreadsheets can be a great source of input to mashups!  A good mashup platform can make a wide variety of data sources (including spreadsheets, HTML pages, SQL databases, web services, etc.) can make all of these sources look virtually similar to the mashup-maker.  (JackBe even has a 'Mashup Connector for Excel' to make this easy.)  Alternately, spreadsheets can be a great destination for a mashup, along with portals, websites, and mobile devices.