The Type Of Tip Most Restaurant Servers Would Rather Give Back
Let's talk tipping. If you're dining in a restaurant in the United States or Canada (this particular piece of restaurant etiquette varies in other countries), it shouldn't need to be said that you're expected to tip your server, to the degree where some restaurants will even force it. However, based on some accounts from servers published online, it seems that some diners need to be told that they should tip money and not whatever random stuff they have in their wallet. (Whether you should tip cash or card is a whole other question.)
While the worst version of this is probably tipping with fake money or a smarmy "thank you" note with some life advice, there's perhaps a more insidious way to tip-without-tipping: leaving coupons or gift cards behind. To be clear, it doesn't seem like this is overly common, but there's evidence that it happens.
As a former server who has relied partly on tips to pay rent or bills, the reason why these are unacceptable tips is obvious: You cannot pay rent or bills with gift cards and coupons. A coupon is the worse of these two. If someone were to leave a coupon offering a discount at some store, this actually requires the server to spend money in order to use it at all, even if it's offering a generous discount. Something like a Groupon is marginally less bad since at least the recipient won't have to spend their own money to use it, but it's rather presumptuous to assume that your server wants the massage or sky-diving lesson on that Groupon.
But what if it's a really good gift card?
So, leaving a coupon as a tip is absolutely off the table, no exceptions, because there's no knowing if it's even of any use to the server. However, I can imagine diners genuinely thinking a gift card is an acceptable tip, perhaps because it has a clear monetary value.
Although a gift card may be less useless to a server (emphasis on the "may"), tipping with one means you're making some big assumptions. For example, a Netflix gift card isn't much use to someone who is using a friend's plan, or maybe they just hate TV and movies. An Apple Store gift card won't be so useful to someone with an Android phone. A supermarket gift card may seem better, since everybody eats, but how do you know that the server lives near that specific supermarket chain?
The possible lone exception is one of those pre-loaded Visa or Mastercard gift cards that can be used almost anywhere — but even then, I'd probably be a little suspicious that you're just offloading a stray card with $5 credit to get out of tipping properly. (On that note, if you're tipping a gift card — and again, you shouldn't — it should be substantially more valuable than the 20% to 25% cash tip you'd normally leave, to account for its limited utility.)
Ultimately, though, if you're dining out, you should be tipping money. If that's not feasible, perhaps skip dinner and use that gift card to buy yourself something nice instead.