Having a second passport provides an option to ditch US passport and stop paying US taxes at any moment.
One basically pays to buy an option to stop being subject to US taxation at a moment of their choosing.
If your assets are under 2mm, or if super liquid and thus easy to pay the exit tax, you have a pretty decent option.
Otherwise, the exit tax can be addressed on a scale of multiple years (Puerto Rico has 0% capital gains under act 60, exemption from federal under irc 933, so move to PR first, realize capital gains, have no unrealized capital gains subject to exit tax when relinquishing...)
I'm not a lawyer; not legal advice, or financial advice, entertainment purposes only.