You never know if one has to be sent in for repair. Also, owning a land machine and a water machine is also very common, as a lot of us hunt both land and sea. I guess a plus for an extra AT Pro would be that it covers both land and water, should one have to be sent in for repair. Then, any extra coils would still be usable. I own 2 yellow ACE's and keep the sniper on one and the large coil on the other for convenience. I know some water hunters who also keep an extra water machine as a backup, as a company building water machines has the usual electrical and coil problems AND issues dealing with water leakage. Metal detectors are not a static piece of electronic equipment like a stereo sitting on a shelf at home, but are used out in crude conditions including sudden temperature extremes, dirt, sand and physical abuse.
