I think it depends on the horse. Most of my horses will get bored working in the ring constantly - they will get sour in the ring, and then misbehave simply for the fun of it. So a trail ride is good for them, they love to see new things (well, Daisy saw a bicycle - a whole pack of them - for the first time, and she didn't exactly love it...) and just enjoy themselves.
But I have one horse who wants a job. I'm actually getting ready to sell her because she dislikes trail riding. She wants to work, she wants a job. She wants to cut cattle - and I don't have any for her.
It depends greatly on the horse and rider combo, I think. If that's what both of your prefer, and she isn't getting bored, then I think you're doing right by your horse. :]