IMO the reason behind mods disabling achievements is to preserve the "value" of achievements. The idea is that, the developers come up with the achievements based on the game that they make, so to achieve them you have to play by the developers' rules, using what they give you.
In this case, this dice rolling thing is for GM mode, right? And this mode is meant to be played by a group of players playing the game together. I don't see why someone would wish to cheat in this manner, when playing with their friends at the same table. Now if they play over the internet, each person at their own home, then I suppose one *could* cheat like this if they wish (e.g. I tell you that I just rolled a 20 and you'll just have to take my word for it and punch in that 20 for me). But I still don't think that's the developers' problem. If this is an optional feature that can be turned on and off, players can decide not to use it if cheating is a real concern. If there are achievements for GM mode, the developers can impose the same rule "achievements are disabled if you choose to use real dice", or something like that.
So the question is whether the developers think it's worth the trouble to add this feature as an option, simply so that people can use it if they wish. Beyond that, if people exploit it in some way when playing with their friends, that's their problem. It's not our place to police how people play their game. They can police their own game.
