I hope they are throwing in rear throttle and brakes too!
Mine does not have instruments in the rear, but the person in the back can see the important ones by looking over the shoulder of the guy in front.
Since my first solo, the rear controls have been used only for a few hours of required dual training at night and in prep for my check ride (oh, and one thousand-mile cross country <g>).
Now that I have my PPL, I don't expect they will be used by anyone at all in the foreseeable future. But I did sort of need them before that.
My tandem Dominator has full dual controls. If I feel comfortable with the individual, I will let them fly the Dom.
The only downside, is if something happens and they freeze on the controls. All the control sticks I've seen are attached with 2 bolts. you could remove the bolts and take the rear stick out if there was any concern.
Whenever possible, I prefer to put the passenger in front and to fly from the rear in tandem seat aircraft. They get a better a view, and you are in a better position to monitor how the passenger is coping with the experience. Passenger head movements will give you lots of non-verbal information about impending airsickness, fear, etc.
I simply would not fly at all with anyone for whom I had a preflight concern about interference with the controls. If a problem does arise in flight, it's much easier to reach forward to give a little distracting tap than to attempt to do anything with a rear seat occupant.
The worst passenger control interference I've ever suffered was actually from a five-year-old in the middle seat of a three-across Bell 47. Flying from the left as usual, I was just about to touch down when I discovered he was fighting me on the collective, pulling up on the dual control which was between his legs . My minimum passenger age immediately went up to 8, and I now require kids to hold both shoulder straps during take-off and landing (sounds like a safety thing to them, but it keeps their hands safely out of harms way).