This argument stems from a RFE that suggest that items could be given to certain companions for them to equip, use or simply carry.
I don't want to clog the bugtracker too much.
It seems illogical or even impossible for certain companions to enter certain places:
The rift required 100 in climbing and a lot of willpower or having to be almost blind.
The quickling tree's entrance is very small.
The tower of eternal flames, well it is extremely hot.
The water dragon cave is airless.
I can accept slaves entering the latter two.