Disease can be transmitted in the fecal stains located on the top bar of frames. It should be removed to help the bees prevent further spread of the nosema disease. Source: Vermont Agency of Agriculture :
http://www.vermontagriculture.com/ARMES/plantindustry/apiary/documents/Nosema07revWP_000.pdf
Since there are no other posts I'll offer my opinion on this one. Take it with a grain of salt, I've only spent a year in the bee business but have read a lot of books on the topic of disease and pest management. There seem to be two camps that surround the idea of transmitting diseases that could be carried by what the bees live on and around:
The first camp says that disease, bee waste, toxic chemicals (lead, insecticide... man-made pollutions), and general gunk from the ground impregnate bee wax and frames. Therefore, you should melt down your old, dark comb and have new comb drawn by the bees. You should also cycle your hive components to keep the general cleanliness of their living environment up to the best standard you can afford. These practices lend themselves to a more health environment for bees. The better environment will help prevent absconding, as the bees more enjoy the quality of live the clean hive provides.
The second camp has used the same drawn comb on frames for more than 50 years. They aren't concerned with the dark color of wax or anything they would deem as cosmetic. This is because function is paramount. If the bees can physically use the piece of equipment, it is more economical to not spend the honey stores on drawing comb. You also shouldn't replace equipment unless it is unable to perform its function. Because of tradition and lack of compelling empirical evidence to the contrary, bees should be reared with your eye on economics.
I can see the merit in both camps. It makes sense that disease lives in filth and you would yourself have as little contact with it as possible and therefore you should subject your bees to as little contact as possible. On the other hand, bees are extremely efficient at cleaning up their environment.
You will have to decide if you're a hobbyist or a businessman; or somewhere in between.