Only the output would be different; all other commands and functionality would stay the same. This would make it easier for players to know whether they should/could receive the message right away (i.e. as something they simply overhear, if they're in a position to do so), or whether they need to work it into their RP because they're being personally approached, either as an individual or as a member of a guild.When you select 'all' as the target for your messenger, instead of the messenger appearing to the pbase as "A messenger waits for the signal to approach Yaddayadda.", it appears as "A town crier can be heard spreading a message from nearby." or something to that effect.
There are some shady edge cases where the distinction might not be welcome (e.g. if you're trying to spoof a town crier), but by and large, it makes a lot of sense to me. There are many ways the system could be further customized or personalized, options toggled, etc., but I'm hoping the simplicity of this suggestion makes it appealing enough to implement.