They are a must in my opinion, you can push more air to the front bedroom, I am actually going to add a second vent in front bedroom to allow even more air to flow, the air shaft goes further than the where the original vent is located.
I'm pulling the unit out of storage next week for a trip, it goes a I believe 2 plus feet but I'll put a measure tape in find out, regardless if you do it verify yours before as they may be different. I'm gonna use one of the old vents with an install ring from the ones I bought. I'll post pics, in the bedroom I kept the original vent style as there was no need to. Lose them.
Farther towards the front/nose of the bedroom vent. Let me know how many feet. Gonna do the same
Mine had 40" depth to the front from the original vent, measure first, make sure no support wood frame is where you hole will be, I barely hit mine, making hole make sure use a tool that won't go deep, use duct tape to seal gap between channel and ceiling where vent housing is mounted and that's it! Good luck!
Youroo!! 
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