I was talking to my buddy and he brought up an interesting point.  If you absolute co-witness your BUIS, what's the point of having the optic?  You're looking through your irons anyway.

I assume you're talking about a fixed BUIS.  You still get at least a couple benefits in using an optic for that setup: a) the ability to shoot with both eyes open, and b) you can focus on the target, and not the front sight post.

Also, when using and optic like an Aimpoint or EoTech that has been properly placed to co-witness, the top half or 2/3rds of the window is free of the irons.  So you can slightly raise your line of sight and look over the top of the irons, giving you a perfect and unobstructed veiw.  This ia how you would use the combo most of the time.  If/when your optic's battery dies or has a problem, (which is usually right when you get to the range) you lower your L.O.S. slightly, and you will be looking thru the irons, never having to remove the optic and possibly loose zero.  (or go home for more batteries)

So while I'm sure you've read how you can look thru the iron sights and dial the dot down to sit on the top of the front post, this is just to sight in, and not the way you would normally shoot.

Tex78