In a workforce divided between a more protected part and a more exploited part (divided by permanent or temporary status, by a border, by residency papers, by union representation), building militancy in the more protected part of the workforce is always going to hit a wall, because the demands of that part are going to be disciplined by competition with the more exploited part.
Just as an example, factory workers in the US can only make demands for shorter hours and higher wages up to the point where it becomes more profitable for the company to move production to the maquiladoras or the Philippines. Construction workers could price themselves out of work if they get wages so high that their contractors will always be under-bid by contractors using undocumented labor.
If you want to build labor militancy that can go anywhere, you have to start by supporting struggles and self-organization of and by the most exploited workers. Those workers are exploited through very direct repression, which is why militant unionism requires defense committees.
This militant unionism and the centering of the most exploited workers frequently challenges existing hierarchies within labor, or is just threatening to administrators who would rather conserve institutional resources than take risky moves organizing the least protected workers and tangling with real repression. So, the defense committee has to be a rank and file institution.