The advise for SharePoint in terms of configuring Application Pools has changed over time. Back in 2007, everything was to be separate (within various limits). Or for “security purposes”, Service Applications had to run under unique identities. But starting late into SharePoint 2010’s lifecycle, with the increase use of BPOS and eventually SharePoint Online in Office 365, Microsoft changed their tune telling us that less was far better. One thing I hadn’t seen was why it was better, and I was surprised by the results. Less really is better!

http://thesharepointfarm.com/2014/08/expense-application-pools/