The 1,8 50mm AFD focuses very fast. If you had a SLOW version, it was probably down to your body's motor. Mine is a D200 and that lens zipped into focus (unless it hunted). In fact, it is hardly a slouch in comparison to the 24-70.
One reason I'd not like to invest in AFS lenses is that motors will break and eventually turn the lenses into manual focus lenses. AFD versions rely on the camera. Manual lenses rely on your hands.
I broke my arm earlier this year and wasn't about to perform the duty of a motor, but today, I am back in the saddle again and glad. But, my 30 year old 50 mm 1,2 is working miracles today but I'm not so sure it would if it relied on any type of motor.
I have never seen an AF-S lens motor "break" from constant use.
I'm curious about the longevity of an AF-S motor too. Knock on wood, my 18-135 will chug on forever, mainly because I don't want to spring for an 18-105 VR or an 18-200 VR. :D
On the other hand, I wonder if the in body AF motor would be more prone to failure because every AF lens needs it to be driven, especially if every lens you owned is an AF-D lens.
Also- I would imagine replacing a body AF motor to be pretty expensive.