If you are talking "tower watch" as if it were a stone edifice with a dozen people in it, sure (but that costs far, far more than the stone). If you are thinking of it as a wooden crow's nest with two in it, then no, not really.
To pull that off, the bandits need a whole bunch of inside information - that the tax shipment is coming, that the bearer of the stone has been set to send a go/no-go message, and so on. A stone given to ye standard farmer wouldn't likely be useful for that, as the Sheriff would ask, "Why the heck is Farmer Jessup trying to tell me about tax shipments?"
Once you have that much inside information anyway, the Stones would not be the real weak link in security - someone's already been captured and forced to talk, or there's an inside collaborator. Stones are then a lesser concern.
The Farmer's stone is probably only good for diversions, "Sheriff, there's bandits over on the east side of the river!" But you can get the same diversion by having someone beat up run into town and say the same thing.