Because you can't name a town where it won't work. That means someone he knows is in every last one of those towns, and out in the middle of nowhere local to the PC, to carry messages.
Right, so it works in the middle of a desert, because someone is going to be wandering by or something, local to him to carry the message.
It's that sort of absurdity which causes us to say no to abilities which say such absurd things happen.
I can't see who you are arguing against but, if we're debating about the Criminal background, I think we should revisit the wording of the Feature (PHB p. 129):
You have a reliable and trustworthy contact who acts as your liaison to a network of other criminals. You know how to get messages to and from your contact, even over great distances; specifically, you know the local messengers, corrupt caravan masters, and seedy sailors who can deliver messages for you. |
Ok, so maybe we should step back and consider a few things about this Background Feature:
1. How does the Feature work?
2. What does the PC gain from using the Feature?
2. How can a DM make this Feature jive with the fiction?
As a PC with the Criminal Background, you have a contact and you know how to get messages to and from your contact. Doesn't mean the contact always has an answer or a way to help you if it doesn't make sense in the fiction and it doesn't necessarily mean that the delivery of said message happens in a timeframe that is immediate or useful.
You also know the
local messengers (etc) that can help you achieve this delivery of said message. Your Background has to do with where you came from
- what you did before you started adventuring (PHB p125). You know the folks
in the area where you were a criminal - maybe that's a village or a town or a city or even a region where you were a criminal. You don't (necessarily) know the messengers (etc) all over the
world. Point is to
clarify that with the DM during character creation - neither the player nor the DM should be making assumptions.
Even if the contact is a
great distance away, your
local contacts can help you achieve the delivery of a message - that is, if you are in that
local area. Beyond that, you know how to get messages to and from your contact. You still might have a chance to do it outside of your
local network if you inquire with the right people. In a village of 30? Maybe there's someone who has sending stones that can reach your contact or someone else in your network. Maybe there's someone travelling to the big city down river next week. Maybe there's a travelling merchant. Maybe there's [insert reasonable fictional bit here]. In other words: this background feature
can be invoked anywhere in the game world with a little creativity but, again, there is no guarantee on timeline to get messages to the contact and there is no guarantee to the actual benefit of getting said messages to your contact.
So... why the pushback when someone tries to use it? It's not the "Insta-win" button that some are trying to make it out to be. Rather, it is a basic feature of the PC that, when invoked, develops their character arc and can either push the overall story in interesting directions or simply provide a fun vignette or side-story.