hm, a nice new buzzword...
But not so new after all. SAP was talking about "componentizing" R/3 many years ago.
First it was "modularization", then "componentization" and now SOA or "applistructure".
It has not happend yet.
I think web-based enterprise portals is indeed a step forward, because now we can "bind together" several applications that publish their UIs as "Webparts" / "Portlets" into the portal and let the portal manage the workflows across these applications.
While this may provide UI- and workflow integration across the different parts of the "applistructure", it does not provide data integration and you will end up with several non-integrated and non-consistent data silos.
Doing the data integration is complex and costly, and doesn't really fit with "on-demand".
So it depends finally on your business and the nature of your business processes. Can you live with data islands? Is flexibility more important than data integration and full automation? Then go for on-demand and "applistructure". If not, better stick with the big integrated system.