It also depends on how the device is protected. On many 8051s you can set a lock in such a way that you can read the device in an encrypted way. If you (or the original programmer) are not careful you leave the door wide open for hackers.
The encryption is done by EX-NORing the original data with a string of let's say 32 bytes. If your program does not fill the entire memory the empty parts of the memory will be filled with FF. All a hacker needs to do is find out if the same sequence of 32 bytes repeats at the end of the memory several times, and the encryption string is revealed!
Always fill the entire memory with random data if you intend to "protect" your chip in this way! This will avoid easy hacks.

Happy programming