Spline does some things nicely.  One of the things I like is that the shapes it creates are inherently mapped (usually decently, too).  You can often export a spline object directly to Poser (just export from Carrara in .obj format).  Given that you can also do groupings and material zones in UV Mapper or in Poser, there is no real need to go into the Vertex Room.

No need but two.  For one, sometimes the spline models are a little messy.  They might have breaks or kinks in them.  I find it easier to clean that up in the Vertex Modeler.  Plus sometimes a model will be more complex, or there are things you just can't do in the Spline Modeler.  It's also easier to assign groups for body parts and materials zones in the VM.  But, really, the main reason I open everything there is for comfort.

By the by, there are ways to deal with the scale issues between Carrara and Poser.  I don't usually bother, myself.  I model at a comfortable scale.  I export.  I go into Poser, load up a zeroed figure, import and drag and re-scale my new mesh to make sure it fits correctly.  Then I export that from Poser, labeling the file "_fit" or "_posed" so I remember.  Then when comes time to rig, I import THAT file instead of the original one.