Not everybody. Some (me) dislike intently the noise you get (HONK HONK) every time you flick it. 'Sides you have to relocate the ignitor (the box). You can either ty-wrap that somewheres, or drill/tap a hole.
Some just remove the rear part, leaving the ignitor mount spot in tact.
Some (me) drill 1/2 dz. or so 1/2" holes in the rear portion (the raised part). That will keep large bodied detritus out of the box, too.
Yes, it will make it run leaner. Unless your bike is spot on already, I wouldn't worry about it. Most bikes are too rich in the first place. Certainly if your bike is in stock trim jet-wise, it's WAY too rich. Don't worry about the airbox mod making it too lean.
***edit*** The regulator of course is the 2-wire one, the igniter the 5-wire.