Dealers love replacing discs and pads, it's easy and good money for them to do.
I have always taken my cars for MOTs at an independent I have known for 25 years. They say that the only reason to change discs is if the structural integrity looks threatened, or if the braking meter shows insufficeient or uneven behaviour
i.e. the big needle on the meter should show gradual increase as pedal pressure is applied, without wavering, and should reach maximum power for the car. Both meters should move together as the brake pressure is applied.
On inspection, scores or groves or rust are no reason to fail a disc. As far as thickness goes, they generally tell by feeling the amount of disc remaining relative to the core of the disc.
In 2010 (the year before I stopped going to the dealer), the dealer said that the front discs were worn below the recommened thickness. I said leave it. When I took the car for MOT later in the year to the independent, I told them what the dealer said, so they got out their dial gauge and said the dealer was only just correct, but they said the discs are not structurally threatened, and said they were ok to pass MOT.
In 2011 (which was the last time I've been to a dealer), same thing happened (along with loads of other supposed problems). Later in the year, took the car for MOT at the independent, told them what the dealer said, independent got out the dial gauge, he said they were worn, but said the discs are not structurally threatened, and said they were ok to pass MOT.
I think I changed the front discs myself in late 2012, just after the 2012 MOT, when the independent said that the discs were looking worn, but still ok for MOT.
So I got a further 24 months at least out of discs that the dealer wanted to weigh into their scrap bin..
edit: sorry about such a looong post, but dealers and discs reaaly annoy me
