My guest would be that they are using the D877.hex in it s raw unaltered form, bearing in mind that dis-assembly is most cases is not legal either. This included two modifications, one for better support when operating the ICD from a supply voltage lower than 4.5V and the other to enable flash upgrades. These of course cannot be updated over the wire, thus the second modification is to replace the 16C73 with a 16F876 which is pin to pin compatible, only in a re-programmable flash version. This enables to user to update the code at a later date. The only reason for making your own ICD is to make some modifications in the process.
Some users are unhappy with the exposed board.
You could quite easy provide mounting holes to enable the board to be mounted inside a small box making the ICD more rugged. Others wish to use the ICD to program 16F877 and 16F876. While you can use the ICD cable and header, and external power source needs to be provided. I ve seen many provide a built in power supply. Speaking of power supplies, if you provide one for your ICD you could look at doing away with the VPP charge pump and also providing the 13V Programming Supply.