I was just looking through a number of photo's and in my opinion, it really makes the engines look out of place against the nice livery of an aircraft. Once the 737-300 and new 737's come along this seem to no longer happen.
I suppose as a kid the airline that his stood out most on to me, was Britannia Airways... I always use to think the 737-200's looked out of place compared to the 757's and 767's that had the engines painted with the cheat lines.
Photo © Peter Birch
Photo © JetTrader
If there a technical reason why the JT8D engines were not painted by some airlines as I notice this is the same for some 727's and DC-9's? For example heat?