Personally I find microchipping horses to be a waste of money. Dgos and cats are different, they get out and loose all the time, they are also small and easily missed. Nor do people react much.
A lost horse ? Everyone reacts, they are kinda hard to miss, the owners tend to be on the phone with animal control and LEO, so pretty much when it is found anyone that happens to have a scanner will already know the owner.
If the horse is stolen, good luck finding it, but even if you do, having a microchip isnt going to give a cop probable cause to enter someones property and check. Most likely your horse is headed to a mexican slaughter plant, or being illegally slaughtered locally. As cheap as horses are right now there really isnt much incentive to steal anything, but even so, who scans anything ? I've got three horses outside, I've never scanned them, have you ? I doubt it. A freeze brand makes much more sense, You can see it from a distance, IE from the road, point and say see that's my brand, also a layman with a Netposse poster can be on the look out for an XYZ horse with a 4D brand. As oppossed to be on the look out for a brown horse with an secret 19834568719779 microchipped code. I just don't see the point.
There is no argument to any of that. Howeverrrrr, after praying really hard the horse didn't go missing just to end up on a truck headed for slaughter, if the owner does think they have found their missing horse and the "new owner" refuses to listen to reason/argument, the microchip can be the final argument of ownership where law enforcement is concerned.
Granted, if a horse gets loose most of the neighbors should know who they belong to but, sometimes horses get loose and travel miles before coming to rest in somebody's yard. That's when a microchip would come in really handy to trace the owners.
To reiterate, microchips certainly aren't the be-all-end-all safety feature to get a horse back but they are better than nothing, just in case against-all-odds the horse is found