Yes. If things could break it would add to narrative depth, more dramatic moments (what if Urist's axe breaks while they're fighting their arch enemy?), and also, later down the line, make the economy work. For an economy to work you need to be able to replace things, otherwise you hit a natural limit of what you need to produce and then you get stagnation. Technically, everything should be able to worn away or broken.
Naturally, it should be a slow process, to alleviate tedium. Even the best things should be able to be worn down over time, it gives them real value: if that adamantine breastplate is never going to break it actually loses some value in the sense that you never have to worry about it. But what if it had hole in it, and that become Urist's Achilles heel? Again, that adds narrative drama to every fight Urist gets into. It would also create a need to dig deeply and greedily. If you know that even the adamantine is limited, and needs to be replaced, what happens when you've dug away all the ore that you are sure is safe, and left with the potential for hell to, literally, break loose? You're left with a hard a choice: Either you continue to dig, or you hang up the glorious relics of the past, and they become a part of the history of your fortress. The Age of Adamantine is past. Stories of the legendary blades and armour live on in stone carvings, but they may never again come. Unless the spire is still there and lets say a particularly bad Forgotten beast comes by and steel won't work. Or maybe you just got a particularly ambitious ruler who wants to raise Dwarfdom up to it's peak gain, to reclaim past glories.... And they order the adamantine dug and the blades forged. Once more, the miners go down into the depths to strike the hard blue earth...