Is there any kind of harm adding a mod to an existing save?

I know this depends on what the mod changes, of course, but let's just assume the mod doesn't actually need a clean save. Is there still any kind of harm done?

I ask because I want to add this mod (http://skyrim.nexusmods.com/mods/7169) to my current character, and I really don't want to start a new game just for this.

removing mods is usually much more problematic than adding new mods, and in the case of La Femme Lycana, it looks like it probably doesn't conflict with anything except perhaps if you have some other werewolf mods installed.