The few that do have it - yes it is very stimulating especially when gently flicked on under head during BBJ - can't really explain the sensation but it is has to be experienced to fully understand how good it feels
One beautiful lady who is now gone/UTR/retired gave simply the best BBJ who had a tongue piercing
One thing I do notice is that several ladies that do have tongue piercings only offer CBJ per their TER profiles which would nullify the sensation IMO
Understand ladies preference with CBJ vs BBJ so not making judgements on their safety/preference

I was pierced all over, but removed several.

I found that most guys found it to be a distraction.  

I still have mine pierced

let me know if you want to try it out!!

I had never experienced or even cared about a tounge piercing.  
Found a HOT lady and while talking she asked if I wanted her to leave it on with a sexy little smirk on her face.
Well she DEFINATLEY knew what she was doing ! Tounge fliking with that ball all over my dick, WHEW !!!
She used it and would run it up and down the bottom of my cock and I actually had to slow her down. It's without doubt a killer sensation if she knows whats shes doing with it !

Doesn't really add anything to the experience, in my opinion.