The problem with peaks are we all have in mind the images of Moria, and all the D&D/DragonLance idea of dwarven cities under the mountains.
I don't remember if in the LOTR Tolkien named surface cities for the dwarves, but the idea we all players have is dwarves should live and work inside of mountains.
If dwarfs can not work "normal" tiles, then they won't be able to grab many of the resources like grain, cows, or even iron. So giving them peaks and taking out plains, or water, in no solution.
Maybe, maybe, let them work peaks +1

+1

and with mining, arete, etc increase the

and maybe gold (let them build mines in the peaks, but they would take many many many turns, as even the dwarves need years and years to build their mines/cities under the mountains).
So this would make dwarves to see peaks as an interesting option, but ... what if they couldn't irrigate? Let them work the plains, so they will have

to keep growing their cities and access to the resources spawn in those tiles, but their cities won't be that big.
And thanks to the peak bonuses, the dwarfs will try to find a balance between peaks and "normal/good" terrain.
Mines in the peaks should have the old "dwarf mine", and so not be pillaged.
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Also, letting the rangers to walk on peaks meant a nasty surprise in one of my last games, when an army or rangers enterered my borders and pillage/took my cities.