SharePoint 2010's search has improved tremendously when compared to MOSS 2007 search. FAST search has more capabilities than SharePoint 2010 search, but these additional bells and whistles comes with additional licensing cost. When you look in to tools and untent-fragment-inner">

Areas where FAST is unique SharePoint 2010 search
  1. Content processing pipeline
  2. Metadata extraction - used for faceted refinement
  3. Structured data search - use Fast query language(FQL) to make sophisticated searches
  4. Deep Query refinement - on the other hand SharePoint provides Shallow Query refinement.Get exact count of matched occurrences 
  5. Visual Search - get document thumbnails and previews
  6. Advanced linguistics
  7. Visual best bets
  8. Tailored search experience based on the user/role
  9. Wealth of API and web service exposure.
  10. Extreme scalability of indexing and querying
So if the business is keen on these unique elements of FAST search then it is recommended over SharePoint Search.

Another good decision point is to answer How many items do you want to index?
The SharePoint index can handle 100M items while the FAST index can handle a virtually unlimited number of items.