All keymoves programmed with advanced codes use up exactly 5 bytes of data, so if you replace one with another, there's no data fragmentation. Learned signals on the other hand use a variable amout of memory, so when you delete one of them and replace it with a smaller signal, there will be a few bytes of data lost. If you replace it with a larger signal, it will need to go to the end of the pile and the entire space previously occupied by the old learned command will be sitting there empty. As you can see, it won't take very long for you to have a very fragmented memory requiring that you erase everything and start over. (Before JP1 I had to do that a few times).

Plus, like John said, if you use learning when you don't have to, you probably won't have any memory left for when you do need it. Remember, there's only enough learning memory for about 24 buttons.

Rob