Tanked water heaters eventually fail (read: leak), it is a matter of when not if hence why they have an est useful life. Water can do a lot of damage in not a lot of time. Things like replacing the rods has nothing to do with the tank eventually leaking. I would completely ignore advice such as "don't worry about it, it won't be a problem unless it happens when tenants are out of town" - uh, first of all Murphy's law says even if that were the case then it WILL happen while they're out of town, secondly water can do damage more quickly than that (while they're at work, while they're asleep, if they're just not paying attention).
If the water heaters are located in an area where a leak would cause expensive damage, I'd look into adding a drain if there isn't one already (not just a pan that overflows, but an actual drain line). If not possible I'd replace them. The alarm is a good way to notify of a leak but doesn't address the underlying problem and is only useful if someone is around to hear it.
Water can do a lot of $$$$ worth of damage.