why to use mp edu and sp edu?

can i modify sp edu to play custom battles?
for example to add no_custom to the regionals units and mercenary ? or will it cause ctd or mistakes in campaing?
i read the FAQ and i know that there is a shor cut to mp but it is a simple question. :lam:

If you read the FAQ you also read about what the purpose of having these two different versions of the same files is? I gather you can draw the conclusion to your own question from that?

If not:

The MP version is for custom battles, with an emphasis on omitting regionals in favour of 'factional line up'. So you get more iconic armies.
The SP version is to give you the full gamut, which, given the engines limitation for custom battle unit-selection, is not suitable to play custom battles with if you want armies as above.
However there is nothing to stop you from modifying the SP version to your liking. As long as you do it 'properly' it doesn't cause any harm. It's different with the MP version, because in order to play RTW against other players you must all have the exact same version. So if your MP edu is different from the other(s) you will be unable to do online battle.