Is making them speak to their family through a metal grate really necessary? Seems a bit much. I don't know how it's meant to be any more pious or devoted to look like a prisoner when you speak to your family for the one time a year you're allowed.
Yes it is necessary for the nuns to speak to their visitors through a metal grate. It shows their detachment from the world. Of course, these nuns continue to love and cherish their families, but God comes first in their lives, and so the grill is a sign of this detachment, and the fact they have left the outside world behind them.
I have been privileged to visit three Carmelite monasteries in Europe, and all the nuns I spoke to, including the late Mother Marie Christiane du St. Esprit OCD, were joyful and happy in their vocations in Carmel.
I hope this explanation helps.