Is a glass enclosing really necessary on a reflector?

You use that kind of reflector with the glass if you are running a fan over the light to cool it. The holes on the sides are for the ducting, and the glass is special glass that doesn't block the lumens. However if you're not planning on air cooling your light I don't see how that would be a benefit in any way.

example:

 

If ur not air cooling the light, the glass will only hold and create more heat,

 

Go ahead and run with no glass and see what happens the first time you do a foliage spray with the light on.

Make sure you cover your eyes when the glass envelope of the bulb EXPLODES.

 

I had a plain lume wing reflector w my first grow and never blew up a bulb, just raise ur lights and be extremely careful if and when u foliar feed, water on a hot glass pane ain't any safer its common sense and precaution.

 

Never had a problem using the reflector with no glass.

 

Only put glass in if you are going to be ducting the hood. There is no need for the glass otherwise. It will reduce lumen output, and you will have to clean it on a semi-regular basis (the dirtier it gets, the more it reduced light output).
Advantage to ducting is more control over your temps in your space, and you can lower light closer to plants to increase light intensity. As you move your light away, the intensity of the light reaching the plant diminishes exponentially, so the closer the better.... as long as you don't burn them