These days 3DMax + Character Studio and a slew of plugins or Lightwave 3D + Project: Messiah and a slew of plugins can be just as powerful as Maya. Poser Pro, teamed with Max or LW won't give you the same quality of character animation, but it can certainly speed up the process by having tons of ready to animate characters and props. For a one-person studio it's a great head start.

FLTrigger, you are exactly right. If you wanted to animate a Poser character running down the street, you would animate your character running in place in Poser and then move him/her down the street in LW/Max. Any props your character interacts with you would probably want to handle in Poser, but something non-poseable like a plane flying overhead you mine-as-well handle in the host app. Check out the pdf user's guide on Pro Pack at curiouslabs.com and that might give you a better understanding how the plug-ins work. Also keep in mind that CL is planning on releasing Pro Pack plug-ins for TrueSpace, Cinema 4D XL and Carrara ... incase you want to be a little easier on the pocketbook.