no border control between Poland and Portugal; they do not therefore check passports/vias
Actually they do. I'm always surprised by people who don't understand the workings of neoliberal governance. Schengen is less about making things easier for travellers and more about reducing expenses.
Most routine border document checks are now down by the staff at the check-in counter. If they don't like your documents they either don't issue a boarding pass or they alert security.
Even the check in machines usually have passport readers.
I've tried to just use karta pobytu at check in in Poland and they always ask for a passport (which I do have I'm not crazy). Coming back to Poland is another story, since most check in staff in other countries just assume the karta pobytu is a national ID.
In non-Schengen using the pasport and karta together makes things faster (since when I fly it's sometimes to places that don't have many Americans passing through).