When to create a Service Application ?

Scenarios where service application development becomes a requirement


Those that share data across site collections or web applications (such as the Web Analytics or Managed Metadata service applications in SPS 2010)

Those that provide specialized calculations or analytics services (such as Web Analytics, Excel Services, or PerformancePoint Services)

Those that aggregate data (such as Search)

Those that are long-running or very intensive processes (such as Web Analytics, Search, or Word services)

Those that are used for middle-tier applications

Reference: http://msdn.microsoft.com/en-us/library/gg193964.aspx