A bit like the whole barefoot caper, there is much argument out there about this too.
The thing to remember is that horses are a semi desert animal and do not perform well when living on green grass. If you wish to maximise their performance, the balance is between providing enough energy for their body to remain healthy under the unnatural work load without compromising their muscles, feet or gut.
Please contact me for a consultation to discuss any dietary concerns you may have for your horse.