The big thing about HDRI is the R i.e. 'Range'. using an HDRI image means that there is more detail thats lost in an LDRI image (ok I'm talk the obvious bit here - BUT what's the impact of this)

the LDRI image will offset it's colour range according to the average colours in the image, losing details either in shadow or light areas or both. So when using LDRI images you will lose details of th light 'blooms' say for exampke you have a grid covered light as the light source, in the HDRI image you'll see the subtle gridding in the light blooms that this would cause, the LDRI image is likely to wash out this detail, or at the other end of the scale - the dog at the back of the kennel resting out of the sun - in an LDRI image the shadow will lose the detail and you'll just see a dark shadow and not the dog lurking within.

The impact becomes most noticeable where real life situations would emphasize the difference, for example the caustics from a glass of water  would emphasize the gridding by way of creating a diffraction pattern based on the frequency of the grid,  (it'd actually be the cross of a refraction and diffraction pattern but I didn't want to get too techy) guess what you're gonna lose it LDRI (not that bryce does caustics anyway or that many prorgrams could create caustics based on an HDRI as photon emitter......)

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Toolset: Blender, GIMP, Indigo Render, LuxRender, TopMod, Knotplot, Ivy Gen, Plant Studio.