Viability of Quick Riposte without using a healing special

This is one reason to why I run Arvis.

As long as you have a healer or you just take no damage to begin with, you should be fine.

IMO, a healing special should be taken if the unit is a mixed tanker (Fae, Nowi, etc) intended for longer combat, but an offensive special should be taken if tanking a single type of damage (Lucas, H! Sakura, Julia, etc), since they will reliably take little to no damage from the source they should be baiting, and once they kill that damage source, their job is done (thus resolving the 'dead skill' issue).

Excellent question and one that needs to be revisited periodically.

Let's start by pointing out that any HP threshold skill is a dead skill slot when you're not in the threshold. Desperation and brazens are kind of useless at the start of a battle. Depending on the character, QR or guard can be useless after one exchange. All skills (fury being a hotly-discussed example) have drawbacks, even bold fighter. (ex: usually means no guard = specials are coming on enemy phase.)

Performance without QR is one reason why I tend not to like slow non-armour tanks unless they stack so much def or res that they barely take any damage (strategy often found on Beruka, for example). A.Tiki is a very textbook example. Her spd isn't like Nowi's or Fae's that can be pushed into the upper 30s or even 40s, or at least, it's significantly harder. If you're not stacking like 44 def (which she does without too much difficulty), two hits, your QR is down, and you're kind of a sitting duck until someone gives you some HP. If you have 37+ spd (which my Nowi does), at least you're exchanging hits one-for-one with opponents even when QR is down rather than getting hit with a barrage of everything. Taking fewer hits also makes easier to regain QR on an aether activation because you're taking fewer hits.

There's a point to it, though.
- Nowadays, you need insane amounts of atk to make vantage better unless you're going for something like deflect melee. Abyssal map units usually have 110+ mixed bulk, so even fully buffed Laevatein is in over her head without a special (and it's currently impossible to combo special spiral with vantage). Most dragons and armours have 80-90 physical bulk and at least 70 magical bulk; melee infantry and fliers generally have at least 60-70 physical bulk (more on tanks) and 60+ magical bulk; those that don't are often firesweepers.
- On short maps (arena, arena assault, PvE maps without reinforcements) one-and-done might be totally fine.
- For units like Felicia or Saizo, QR might seem strange to some people, but the fact is that offensive units can be so fast that you would be happy just to avoid their double. Neutral base kit B.Lyn will initiate with 48 spd if she has hone cavalry. Nino+0 +spd with fury has 42 spd, which probably won't double you if you build properly, but if you want to double you'll need a lot of work if you don't have QR. There might only be one opponent that fast, so you can use up your QR on that opponent and natural double the others (because you were competing with that speed demon, so now you're actually really fast and doubling everyone else), or you might even be in that scenario where that's the only opponent you need to fight on that map and your teammates will deal with the others.
- Healers, including Falchion wielders. To give you HP.
- Absorb+ healers using QR such as Lissa. That's basically the same idea as using a healing special, but done differently.
- If you're not using QR, what are you using? Guard, which is even easier to lose? Renewal, which is really slow? Breakers, which are extremely specific? Chills, which only hit one opponent and are therefore very limited on any map where getting knocked out of QR can cause a lot of issues? Def/res link is pretty hard to get. I would say you're better off accepting that you're one-and-done, using a healing special, making sure someone can give you some HP, or building to resist all damage and keep your QR indefinitely. The only option I can think of which is especially convincing is special spiral. Like, slaying weapon + DC + ignis or something like that? That's a completely different tank than a QR tank; this is OHKO or bust, except with a lot more bite than vantage builds and the 30+ extra damage from the special really can get you into that 70-90 potential damage zone that will get through opponents in one hit outside abyssal maps.

I've used QR on Felicia since the day I built her and only recently experimented with desperation for abyssal maps, and it's pretty solid; just slams the door tight on WoM dancers. I think QR has a lot of potential combined with the desperation weapons released last summer, especially with brazen atk/spd. It's one way, a rather fancy one maybe, of getting brazen+desperation going without being kind of dull at the beginning of the battle.

I don't run Roy with a healer. He just doesn't take damage most of the time.

Yes I think that's the general implication.

Unless you're building for Chain challenges I don't think it matters if it's a dead slot after the first use. A Skill only needs to trigger once to totally swing the match.

That said when I use QR I usually try and incorporate some healing mechanism into the team.