by stev4uth
As noted so far, they can be pretty good. Where they really can help is generating income so that you can grab items from the center section. Nothing can mess up your strategy than a few bad dice rolls and someone else grabbing an important tile (especially ships or castles).You can of course forego mines and focus on getting good bonus tile options. But those are not always a sure thing since, depending on how many people are playing, the ones you know might be big on points may not come out. That leaves way too much to chance, but some like to live on the edge.
I think it also depends on what is going on with the other players and their choices. If you see that you might be going head to head on items, having mines to give you some extra silverlings will help you as the game progresses. Even closing out those three spaces in the base game will give you a bit of a point bump as well early in the game.