UK should be barred from joining
The system is to strengthen the border controls for third country nationals who need a visa to enter Europe.
UK requires all third party nationals get a visa, even ones legally resident in the EU who have free movement within the schengen zone and a residence card.
So they should be turned down, until they fully implement the 'EU Residence permit as visa equivalent' rule.
Letting UK join would strengthen the barrier between the UK and EU for third country nationals married to EU citizens. It would make it harder to cross the English Channel. That is COUNTER to the EU treaty.
In other words, they should insist on some sort of quid pro quo.
It's insane that anyone who is legally residence in Europe, and has all the permits, and has the legal right to travel, still need to get a visa to travel to the UK.