[Agrajag]

There's no "modifying of files" going on, there's a text files with the prefix .dm that needs to be put in a folder, and then a few more clicks to enable it in-game.
No modification of any sort, and all changes are reversible with a click of a button.
The option of mods is available to everyone, all you need to do is download the correct patch (in the case of DOM2) and you get the modding manual in the /docs folder, and you can easily use that to create a one-command-line-long files and enable it, no visiting forums required.
Its also a much better way, because in HoMM3 you auto-downloaded the maps from the other player, so you had no idea what that map contained (the other player could have disabled spells you didn't like disabled, or enabled spells you'd rather be disabled, or just make himself start with 10,000 Black Dragons), in Dom2 you could just open up the .dm file and see what it does.
And even if you choose not to auto-download, but rather have the other player presend the modded map, you still have to face the "difficulties" of locating the correct folder and putting the file there.