Technically mutation is just a genetic change. Not illness. A black person with blue eyes is also a mutation. That's not an illness. Mutations are normal. Sometimes they give you an extraordinary feature, sometimes you just grow an extra limb.
The problem with the limb thing isn't the extra limb itself, that'd be awesome... the problem is going to form in a spot where it's either useless, compromises the functions of other parts, or fits properly but is vestigial so it's just like.. durrrrrrrrrrrr.
Be careful what you wish for
You probably couldn't handle it
Don’t worry it’s an advantage. That means it’s ok
-Hey buddy, they didn't mean you. I love you and you make the best coffee.
"Hey shut up we're perf" replied Johnny Long Cock.