[Lizard]

I'd like some "real" users to respond to this post too, but I'll give you an Omni perspective:
1) We have users who only find use for one of these three apps; we have users who rely on all three daily.
2) We intend for the three to work together well (via import/export and LinkBack).
3) Some of our users were using a combination of Outliner and Graffle to do project management, and were begging us to develop OmniPlan, so they could have an app that's really designed for that purpose.
3) We don't really want to merge any 2 of these apps, because the UI would just get more and more complex, and harder to use. And we definitely have users of Outliner and Graffle doing things that would not fit in OmniPlan very well at all, like writing books, creating slide shows, drawing floorplans, etc.

-- If you're just managing your own "To Do" list, you'll probably find that OmniPlan is overkill and you're happier with OmniOutliner (or OmniFocus, an up-coming app based on GTD-like principles).
-- At this point you can export from OmniPlan to OmniGraffle to get WBS diagrams, but we do hope someday maybe we can take a Graffle WBS and turn it into a Plan.