I think it is a great idea! I actually plan to have mine done when they get their coggins done next spring. All of my dogs and my two parrots are microchipped so why not the equines?
I know someone who had to identify her horse and show proof of ownership (fire displacement) and she lost all of her coggins paperwork, pictures, everything in a fire. She got him back because she had friends who had pictures, but the vets there recommended microchipping even if it is for nothing but pure proof of ownership. If your horse gets out and someone captures him and is claiming ownership as well, think of it as his permanent ID tag!