For the ones who ride outside a lot:
For the ones who are experienced because they ride indoors, some say it might help with core but pilates classes would be better.
- More aerodynamic. Oops, that only works outside.
- Might put them into a position that is more 'natural' for them - closer to what they ride outside.
I believe that it is really just a matter of what is comfortable. Outdoor (road) riders likely will feel more comfortable with lower bars. The casual urban beach cruiser rider will be more comfortable with higher bars.
What I look for is the overall body position. Because I teach on the Keiser M3s at two facilities, higher handle bars mean a longer cockpit (distance between the saddle and the handlebars). This will leave some riders too stretched out, with an angle greater than 90 degrees at the shoulder. This can cause upper back, shoulder and/or neck pain. Also looks like they are leaning out a window about to pass a tray of food to someone in a car. Barring lower back problems (always check), I bring the handle bars down to the point where they are less then 90 degrees when they reach for HP 2.5.