I just don't see it. I mean, they're cute and all, but given some of the horses I have known versus dogs I have known, I think I would opt for the dog if I lost my sight.
Even a SMALL panicky horse is a dangerous object. It took three of us (vet, farrier, and yours truly) to hold a very small mini for his shots. And horses are prone to panic. The best horse I ever had was the most bombproof equine ever, but when she DID panic -- look out! Probably has something to do with their being Movable Lunch on the Prairie.
They also can't really be toilet trained. When they have to go, they have to go. I've had horses that didn't like to dirty their stalls and would always go outside, but outside had to be available - if they were cut off from their paddock they would just let fly where they stood.
I just don't see it. Too bad, 'cause it's a cute idea.