John Andrews of Microchip says:
...The RB3 pin is only used for low voltage programming. While the chips come from the factory with this mode enabled, in case you want to use it as your first programming method, the ICD (and all other commercial programmers that I know of) use the more traditional 13v, programming method. So when you debug with the ICD its important to disable this mode in the configuration word unless you take the appropriate steps to prevent an inadvertent high on RB3, which will cause the device to stop running and go into programming mode.Making this connection to the ICD isn't required since the ICD uses the 13v method. Its not recommended since it uses another one of your I/O pins that otherwise you could use for something else.
If you intend to make use of the low voltage programming method for a field programmer, then you will need to take the appropriate steps to insure that this pin is normally pulled down to keep the device out of programming mode. Then your programming device can pull the pin high when you want it to program. Even in this situation I would not connect RB3 to the ICD as this might interfere with your custom programmer interface.
I have seen others use the low voltage method to allow other chips in their system to program the PIC with 5v only signals and also use the ICD with its normal 13v programming method for s/w debug (in this case don't disable LVP)....
Interested:
David A Cary Says: " Jim Robertson has a few more details "
Questions:
I use ICD 2 as a programmer to program PIC18F8720,
Microchip also has the PRO MATE II, does anybody know the difference between these two programmers to program PIC18F series?