Your question is pretty wide dude
HDRI, in the very first approach, helps you setting a much more "natural" variation of lighting. Cause these are directly taken from a real situation/environment. Adding also it helps really producing accurate reflections within reflected light levels, as these lighting points are oversampled within HDRI maps. But also cause you then got a "full env. dome" for reflections.
Usual reflections within a Chrome shader e.g. won't give you a real-looking result, until you set up the HDRI in fact. Car-concept modelers shouldn't show renderings without HDRI. Without HDRI, car paint is just dull. All car marketers play mostly with the cool effects you got in reflections.
Talking 'bout chrome or car paint cause HDRI is mainly effective with reflective and glossy materials, as these mainly exist through a environment. It affects all materials of course, as standard lights would do.
Well, in some way, think about HDRI as a way of not setting up a complex and natural skylight. Just to compare, a normal skylight should be set with different lights colors in order to get a result close to what you get with a simple hdr file.
So, to me, it's used, as soon as you can use it. Cause it pushes all one level up in your renderings. Whatever the situation. As Ambient occlusion does. Once you start with it, you're addicted at first rendering.
Both are keys, as you said, to get improved photorealism.
Well, I would say no. Well, surely not as much as photons or high sampled depthmap shadows can be I mean. It's just a 1-2 megs file, opened once the rendering starts. So with reflections, it's just as raytraced reflections are, a bit heavy, bot not more cause it's hdr file compared to a scene object. About lighting side, it'll depend mostly on your GI/FG settings.
For outdoor modeling, I would say, always.
For indoor, mostly when you got reflective shaders and/on curvy surfaces (cause there's the real killing effect)
And as Tim told ya ... it gives you great preview rendering even with non textured scene. (prevents setting up a 3 point-lighting, which doens't give you a feeling of volume, as hdri does). 3 point light is cool start, but well, you'll notice the diff. quickly.
Also good to know that future VideoGames engine will use it thoroughly. Some engines already do I think.
WETA plays with it all the time, for sure (I-Robot, in every frames).
PIXAR and Dreamworks don't need it at all, but prolly use it (sss is much more usefull with a Shreck or Tilt than some hdr map).
So it depends on the realism and look needed.
Good and bad reasons using it, or not
Lol, that i can't objectively answer. I love HDRI too much. All I can tell so far is that I quit another modeling tool cause I really needed HDRI badly. So my reason using it relies more on a soft dealing correctly with it.
It's also not producing sharp nor accurate shadows.
You'll just get some ambient occlusion effect, through the FG, but HDRI shouldn't be used without any real shadowing light for a real looking rendering.
Hope you got usefull infos in there.
Try it, love it, adopt it