That's a good point- I use Word mostly for word processing, but I have WordPerfect too. Not very long ago I got sent some documents (legal, no less) in a format specific to WP. Is one better than the other? Well, I know how to use the one a lot more fluently, but for that document the other was by far what I needed.
I have both Poser and Studio, I prefer Studio's interface and rendering.  I also like the D/S specific coontent (used it since the beta where ya got lo-res M2and V2 with all morphs, so that is nice). I like the dynamic features, the python plugins and the better animation of Poser.
If I want to create a landscape in Bryce and populate it with people, I will definitely use Studio- it just works a lot smoother. If I want to build some object in Shade and put a person in the scene, I will definitely use Poser and the Poserfusion feature.  To do otherwise in either situation would be basically dumb considering how the programs integrate. I have, like and use both. I'm happy and grateful that there's so much overlap with the content, because that actually is what I really like.