Just because your girlfriend isn't taking advantage isn't the real issue. If I were a guest and got cleaned out, it would be a thought, and I probably would not return.
I also have not returned to games that regularly allow...
- rabbit hunting
- players to discuss their hands
- string betting
- tournaments using dice chips
- self dealt games where players to choose not to cut the deck
So the cell phone thing - it would be one more way that I could see people I didn't know very well possibly cheating me. Money, even $20, is something that someone out there would try to steal.
You can allow it if you like, but before you get defensive, be aware you are on the opposite side of the rules.
Then again, I can go 2 minutes without being on my phone, and I would never be as rude as to ignore the table to play a phone game. I guess I'm just don't have the Millennial mindset.