Potlet - IS Architecture

Hello i have a little confusion about the portlet and IS and how they fit together, if you clarify this for me that will be super, and f you explain why that would be the BEST,
my question goes like this, i have read some of the doc of IS and portlets and i know that designer plugin in the eclipse IDE offered by SAG give the possibility to create simple screens using only drag-and-drop and that is fast and great but my question goes like this:
I have seen some sample projects that link the screen fields of a given portlet directly with client WS inputs and then they implement the logic of the service in the IS using flow services(drag and drop of branches, sequences, loops…) which is ineffecient in a real world and does transforms the IS role from routing messages to holding logic and business inteligence using flow services , is this the best approach, and they use flow services like functions that get data from database and loop over result and maybe call other service and all this stuff It seems like working with functions do you recommend this approach or should the portal APP access the database directly and do all what it need to do directly like a stanalone Webapp and use IS only when it need in to react with other system.