At some level, this is a depth v. breadth conversation so I probably start by segmenting your audience in terms of anticipated usage. Obviously, there are some benefits to having an all-in-one application but you’ll also find associated costs – both financial and otherwise. And if 80% of your users are focused on a single function, it may not be smart to compromise the application by supporting additional requirements. By the same token, OMB has launched an effort to get rid of duplicative websites and I’m sure that we don’t want to be doing the same thing in a couple of years for duplicative apps so figuring out how to consolidate your requirements is important as well.
It was a slightly broader topic (web portal v. just mobile), but interesting blog from Gartner on efforts to create universal portal for all services within the U.K.