Your idle RPM drops because the alternator has to make that extra current from somewhere - its not a magic device, it converts kinetic energy to electrical.

Need more electrical energy? It will tale more kinetic.

For the light dimming, that could be either that the alternator isnt fast enough to respond to the power demands, or the power demands are so great the alternator cannot produce enough power at idle. Neither one is a big deal unless your car lights dim while at 2k rpm.