Good question. IMHO, our headrests were designed that way by safety conscious engineers. I am pretty sure that studies have shown that most customers do not have their headrests in the proper position relative to the back of their head. I have kids so putting them all the way down is better for them. For those without rear passengers to worry about, removing them actually improves rear sight lines for better safety. If you were building our rear seats only for folks larger than 5 feet, their limited range of motion probably is correct.
So the short answer is that it depends on what you use the rear seats for and to remember to adjust headrests for your actually passengers. Don't forget to adjust yours and your front passenger.
Jeff in NYC
MB '17 B250e; MB '06 E500W; MB '81 380SL