Depends on how badly its damaged......If you are talking about 30 percent of the decal missing.chipped off, you might start considering replacing or if possible, restoring it. But I think if the damage is less than that, you can try to just live with it as "patina" as others have already suggested.
It is also subjective to where tha damage is located. If the damage is small but happens to be in the most important/significant part of the decal, a replacement/restoration might be well justified....
JMOs