As a rule of thumb, I look at the brake pads lining material and compare it to the pads metal backing. If the brake pad lining material is 1/2 the thickness then it is time to replace the pads. If the lining material is less than 1/2 the thickness, then you have waited too long.
Your SEL may have different specs so you should look into getting a manual or CD as reference. But this is what is listed for my 400e:
Rear pad lining thickness (new): 11.0mm
Rear wear sensor trigger: 2.5mm
Min rear pad lining thickness: 2.0mm
So if your SEL has a rear wear sensor and it is working correctly, it should kick in well BEFORE the minimum thickness is reached.
Since they say the rotors are worn, then you might as well replace the rotors and pads in the rear. Not a difficult job.
Hope this helps.