My (perhaps flawed) understanding is that with IES, you can have a room with a lamp and put a 'real' GE 100 Watt Soft White bulb in it or an array of Sylvania Grow Lites in your underground marijuana farm. Something else might yield equal or superior results. Obviously, if you're doing arch/viz then having the realworld reference is important since you can give clients a more realistic idea of what the space will look like with the specified lighting.

I suppose how important that is for Poser depends on how exacting someone is in terms of trying to recreate a real space. Some people who restore cars want exactly the carb that matches the build date of yada yada because they changed in the middle of the production run... Most folks probably aren't that picky about the lights I'd imagine as long as they look good. Perhaps for photographers or other people experienced with lighting having digital versions of the setups they're familiar with would be very desirable - assuming of course that they would actually get the expected results.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken