I think it's better than nothing, but the best is still for them to have a real companion (a friend, not et bully that bites and kicks all the time).
Two years ago, I put my horse in a livery yard where he couldn't share his paddock with another horse. In the end, the yard's owner decided to let him stay with his cow because he was happier that way, but still, he looked fine and all, he had no problems, but when he was back home, in field with other horses, I felt he was better.
Of course, when they are on their own (even with others horses beyond the face), they are happier to see their rider and to do stuff with him.
For exemple, I know that my mare feels good... when she doesn't call me when I arrive. She used to do it all the time when she used to live alone, and with an other horse she didn't like. It was good for my ego, but later, when she was with other horses she like more, she started calling me less and less... and it's a good thing because it meens she has a real horse life.
However most of the time, we do the best we can, and the best we can is not always the best we wish. As long as the horse seems to be ok with it, there is no need to be too worried.