Let's imagine a lvl.7-10 player in a Grotto. It's a very intense and hard location, requiring concentration of all player's efforts and quiet a lot of luck (just as a roguelike should be!).
But once the player gets to the walls and starts mining them it's just becomes a ton of "Free resources". Enemies cannot attack the player from many sides, so in general it's safe. When in a narrow just-dug-up corridor, it's absolutely safe. And there are plenty of ore and gem deposits around just waiting to be mined (packing up full twice in one go).
Basically this "drops" the value of gems to zero, which are extremely hard to get otherwise (e.g. during a Dungeon/Tomb raid). If the player is lucky enough to survive the entry location of the Grotto and get to the wall, he will gain access to almost unlimited amount of easy resources.
(UPD) Such free resources may be fine for Orcish Mines (i.e. end-game and extremely hard to get), but not in Grotto.