I've seen this question come up occasionally and most people say it's unnecessary. I've always needed one to travel solo with DD to Latin America, but never to Europe. Anyway, FWIW, DH is on his way to Canada with DD today and they were asked for a letter with my permission when boarding the train. Luckily, he has until they reach the Canadian border to produce the letter, so I am printing, signing and scanning one with my authorization now.
This was the absolute last thing on my mind this week with a newborn in the house!
When I was detained with my sister, it was also Canada. We were traveling with my dad, and she and I were questioned separately from our dad. My sister was really scared.