The needles are pretty much identical. The only difference is that the needle for microchipping is a slightly bigger gauge. However, microchipping has a very useful purpose. Tattoos as well, but they fade, get covered with fur, and are sometimes hard to find the owner's info through. A little pain comes with these procedures, however, it's over shortly. A little numbness for a day or two and the dog or cat is fine.

Piercings, however, would always be exposed. The piercing would get knocked around constantly which would risk it getting pulled out (additional, greater pain). If by some miracle it didn't get pulled out, it would take a long, long time to heal (if it ever healed at all). Dogs get in bizarre places, like to roll around in god knows what, thus exposing the wound to a plethora of bacteria and potential infections.