You do not have to take a voucher or miles for a lost bag: the rules entitle you to money. Why airlines offer them, what to reply, and when one is worth taking.
Vouchers, miles and travel credits are cheaper for the airline than cash, which is why the first offer is often made in them. Nothing in the Montreal Convention or the DOT rules lets an airline settle a baggage claim in vouchers unless you agree.
Reply in writing. Thank them, decline the voucher, restate the amount and the basis, and give a date. If the cash offer is lower than your documented loss, answer the specific reason they gave: if they applied depreciation, point to the items under a year old and the categories that count at full value; if they say a document is missing, send it; if they cite the old limit, send the current one.
If the second answer is still short, the next step is a complaint to the US DOT (for any flight touching the US) and, for a clear-cut amount, small-claims court, where airlines almost always settle before the hearing.
Work out your own claim → Two minutes to your amount and deadlines.
Because they cost the airline a fraction of their face value and most of them expire unused. A $500 travel credit that must be spent within a year on the same airline, on fares that are not the cheapest, is worth perhaps $200 to the airline and often nothing at all to the passenger. The liability rules are written in money for that reason.
Three sentences. Thank them for the reply. Decline the voucher and restate the amount claimed in money, with the provision it rests on and the date the answer is due. Say that if the amount is disputed you want the specific items and the basis in writing, so that the dispute can go to the Department of Transportation. The claim builder writes this reply for you when an offer arrives in your case.
Sometimes, if the cash offer is close to the documented loss and the voucher is a top-up you would actually use, with no expiry that bites. Value it at what it will really save you, not at its face. Never take a voucher that is described as full and final settlement of a claim the airline has not paid in money.
Usually, yes. An airline that disputed the claim would say so; an offer of any kind is an acknowledgement that something is owed and a negotiation about how much and in what form. Treat it as the opening of the settlement, not the end. Keep the offer letter: it is useful later, with a card-benefit administrator or an insurer, as evidence that the airline accepted the loss.
Nothing that helps the airline. The Montreal Convention makes the carrier liable for damage sustained and limits that liability in Special Drawing Rights, a unit of money; the DOT rule sets a minimum liability in dollars. Neither mentions credits, miles or vouchers, and the DOT has said that airlines may not require passengers to accept them in place of the compensation the rules provide. A voucher is a business offer, and you may decline it, in writing, without giving a reason beyond the rule. If the airline insists the voucher is all it will offer, that sentence in writing is exactly what a DOT complaint needs, so ask for it.
Ask for the arithmetic. An offer of $600 against a $2,000 claim is usually a depreciation table applied to your list; the rules do not impose one. Answer item by item, concede the one or two items where a deduction is fair, and restate the rest. Give a date. A second low offer is the point to file the DOT complaint and say so.
Yes, as long as you have not used it. Say so in writing and ask for cash instead.