Miners and fishers don't have the inability to talk, its only the crazy ones that don't talk. If you've ever been mining or fishing with me, you'd see that I always drag at least one friend to go with me, and I like the chatter the whole way. When conversation gets boring (or if its late and im tired), I'll zone out and watch a movie or sometimes I've even used my free hand to text while mining.
Since you brought up AFKmists, I'll also say that sometimes I sit in pront with my hom out, or even in the Hidden Maze (mostly jur[3] farming for my sin) while I sit doing my homework and feeding my hom and letting it kill. I'm still there at the screen watching my hom (the stupid hunter fly has tried to ambush me several times >.<;

and feeding it myself. Usually i end up chatting on main instead of doing homework, but the point is that I'm still there, feeding the hom myself, I'm just multitasking, and multitasking is no where said to be against the rules =P
There are disadvantages to miners/fishers who multitask. You run the risk of zoning out and letting your gold/OCAs fall onto the floor, for someone else to take or for it to disappear if you don't zone back in fast enough, or you might not notice when a random monster is killing you. Or if you really zone out, and don't come back when you're bot tested, that could be bad. So there is a certain amount of minimum effort you have to put in, or you might be facing bad consequences. Plus the risk of permanent damage to your wrist.
But if people want to be crazy and mine really fast, I think its their right to do so. I personally find it boring (even while multitasking) and cant stand to mine and fish much and usually only go when I have a specific goal of things to get, and I'd rather spend my time partying or doing something actually fun when I'm playing a game. I think there's enough reasons out there not to spend all your time mining, that having it take longer through npc speed isn't necessary. (The idea stated early in the thread to keep the npc slower so that macros aren't as useful, but isn't that noticeable to actual humans would be nice, if it could work.)