Since I started using indirect lighting I have noticed my figures and hair props coming in as light emitters.  After a day or two of blown out renders I noticed even my ground plane was set to be a light emitter.

Is this somehow related to enabling indirect lighting, and is there any good reason for this? 

      That box might better be thought of as "participates -be it either passively or actively- in indirect light calculations".

     Hair would participate in IDL by passively reflectling/diffusing a fraction of the light hitting it.  Pixie dust would participate by actively casting light.

     The default is rightly that everything participates in IDL calculations.  The only exceptions would be for certain items with unusual properties (a background billboard, e.g.), or disabling transmapped hair IDL participation in order to save on system resources for marginal computers.

if your object surfaces glow, they may have excessive diffuse + specular + transparency + reflection + refraction, e.g. greater than 1.

Light emitter is a completely WRONG name and this creates confusion.

All this checkbox means is that , as in real life, every object that recieves light does diffuse that light into its surroundings. => Creating In Direct Light.

Turn the checkbox OFF, and the object does not diffuse incoming light back into its surroundings.

A better name would have been Light diffuser, or Indirect Light Diffusor.

As all Real life Objects do....full time.

Ony a black hole in space is a NO Light diffuser.

Your conclusion and findings are corrrect.
With IDL ON, you need far less Direct light then without IDL.

Total light in the scene, is the Direct Light + the InDirect Light provided by all objects diffusing the Direct Light.

Oh yes, and one last remark.

Defaut is : light Emitter ON for all surfaces and objects.

Exactly as in real life.

Well it certainly seems that objects that should be blocking light are emitting it.  In places where I'd expect occlusion (guess I could turn that on) I'm seeing glowing.

Maybe there is an ambient component I did not check.

Either ambient occlusion in lights or material.  I would check Min rate bias on your lights first and set them to a low value (i.e, 0.1).

another disturbing effect i have noticed a couple of times, is, that loads of material shaders have an "alternate diffuse" texture or color added which causes the closer objects to another object to be emmitted heavily with light (ie hair hanging close to the head causing a fraction of light close to it...

I usually turn all this things off....