Depends if your horses are used to constant shelter or not. I've seen horses never use a lean to, in favor of standing in a group, or maybe standing under the trees. And the only time they did use a lean to, was because that was where they were fed and they were only looking for dinner.
I'd say visit the place, and see how it is. Is the wind bad (I wouldn't mind in the summer, but that same wind can cut like ice in the winter)? Are there tree lines to break it? Are their any trees to give some semblance of sun cover in the pasture? If yes, I wouldn't worry about having an accessible man made shelter in the pasture. I'd worry more about how well they got along with the rest of the herd and if I would have an injured horse the next morning.
Think of it not as a failure but as a success in how not to do it.
Don't look in a horses mouth for a gift.