If you're not going to be deactivating and reactivating them in a hurry this isn't a problem . . .
Just to clarify for the OP, having individual dwarves switch from "I am a Miner" to "I am a Fighter who swings a mean Pick" and back all the time is generally a bad idea. That's due to the hardcoded Miner "uniform" which consists of:
a) a pick, of any type, and
b) absolutely no armor.
That's why laularukyrumo said to
never re-enable the Mining labor on those who you've chosen to draft, because as soon as you tell them, "picks are for digging," the first thing they do is get naked. Now, good Miners
are in fact pretty badass about defending themselves, but if you ever want them to go and
attack anything, first they have to take a day or so to collect & equip bits of their military uniform--all the while, your civilians are being killed.
As Bandreus said, only the most extremely bad thoughts (like a tantrum spiral) can affect a warrior. Once they get a couple dozen kills (easy if you use cage traps & pitting) under their belt, they "don't really care about anything anymore."
Getting high-level Miners actually
doesn't always require a lot of digging. Sometimes they migrate in, sometimes they get moody. Although, I doubt either of these methods provides the (apparently rather negligible?) stat gains earned through actual digging, so they might not be preferable for warriors-in-training.
Digging sand/clay/etc leaves no byproducts, & therefore causes hardly any FPS drain.