Their the meat shield for the surface against Darkspawn.
At the very least they need to be supported so they don't all die and the Darkspawn block the Lyrium trade.
If they are the sole bulwark against the Darkspawn, then they are doing a shoddy job. Just look at the Anderfels, who have to contend with Darkspawn as often as the Dwarves. And the Lyrium trade has kept flowing for hundreds of years without major military intervention. Evidently the monetary support the Dwarves get through this trade is adequate for them to shore up their own defenses.
Moral, practical, strategic, and self-interested reasons.
Morally, no one 'deserves' to die to the Darkspawn. I would hope this wouldn't be controversial. Not the moral, not the evil, not the innocent, not the corrupt, not the very top, not those at the very bottom. The Darkspawn are not some karmic moral-retributive force, and even if they were I would still disagree with it. If you care about any dwarves below the surface, including the casteless, you should be willing to match that care with aid to help them not be destroyed by the genocide that is Darkspawn expanion.
Practically, strategic, and self-interested reasons blend together, but basically the Dwarves are the biggest hot-spot and focus of the Darkspawn attention in the years of peace. While I doubt the Dwarven claim that Orzammar serves as the barrier between the Darkspawn and the surface (there are plenty of other exits from the Deep Roads other than Orzammar), aiding the Dwarves does mean they will fight a conflict that you will not. It does seem to help keep the Darkspawn distracted away from the surface, which is good.
There's also the point that the Dwarves are pretty much the sole supplier of lyrium. This isn't necessarily the case forever- but no one has exactly created a human mining industry to find the stuff either. Lyrium itself is a strategic resource that, regardless of all other thoughts of the social issues of magic, is a potent tool for both bolstering magics (where the only alternative is the riskier blood magic) and in mitigating magical threats (through Templar-like abilities). Lyrium is not just a strategic asset, but a major factor in social stability and addressing concerns of out-of-control magics.
Aiding the Dwarves or Orzammar so that they don't lose their fight is simultaneously a morally good deed, a cost-saving alternative to Dwarven defeat, and an enabler of social stability at home.
There are Dwarves outside of Orzammar. Dig a hole deep enough and you will dinf Lyrium. WHen you find Lyrium hire surface Dwarves.
And I don't see it as a morally good deed, beyond that helping ANY person is a good deed. I see it as completely foolish. I wouldn't send a man, who insisted on living in an igloo in Sahara my precious water, just so that he could make more ice.
If the Dwarves insist on living udnerground, the natural habitat of the Darkspawn, then they will have to contend with the Darkspawn for the living space. Simple as that.
Well I guess if you look at it one way then no one would be deserving of aid; not the humans, not the elves, not the Mages, not the Templars, etc.
I would say that out of all the races of Thedas the dwarves are the most threatened with extinction.
They are indeed, and they've brought it upon themselves.