i'm not bagginsbill (or even a  lesser substitute), but the following are problems i've read about and observed in the forums with Poser IBL being directional light::

it doesn't affect specular properties
it doesn't cast shadows
it's very inaccurate and low resolution, so you won't get the same precision as with a directional light

i've been using Synthetic.'s iterative process, and starting with a plain IBL of 10 to 20% with my directional lights, and making a new IBL, then using that to generate the next IBL, and keeping my directional lights.  i think that process has worked pretty well for me.  the only downside i'm experiencing so far is that i can't capture anything in the center of the stage.  soon i'm going to make an image of a woman in a red dress in a white room, and i'd love to see the red on the walls and such.  there's probably a way to do this, but i right now i'm working on another image.