They are not a guaranteed return because you can't be certain they will in fact make the difference between "teammate on the ground" and "teammate up and fighting" for at least 1 round.
Whack-A-Mole isn't a guaranteed return in all circumstances either. It's only guaranteed in a few very particular cases.
1) Enemies ignore downed allies and don't hit them with AOE's
AND
2) No enemies go after you would heal a downed ally before he gets his turn.
If those 2 conditions aren't met then whack-a-mole isn't guaranteed.
That is the virtue of the "whack-a-mole" strategy: If there are no enemies between you and your teammate in the initiative order, you know with 100% certainty that your healing spell will purchase at least 1 round of actions for your teammate. And if there are enemies between you, you know that, and you don't cast the spell in the first place.
This also assumes enemies aren't particularly vicious toward downed PC's. If they are then the PC might be dead before you can pop up the whack-a-mole.
However, an investment does not need guaranteed returns to be a good investment. It just requires a risk premium: The greater the uncertainty of the payoff, the bigger that payoff must be to justify the risk.
Sure. But that also applies to the downsides and not just the advantages. Suppose you use a high level heal spell in a scenario where the PC wouldn't have actually been downed if you didn't use it. What's the downside?
Likewise, suppose you didn't heal a pc before he dropped to 0. What's the potential downside.
It's not just about the potential benefits, it's also about mitigating the potential downsides.
My contention is that potential downsides of not healing as I advocate are so bad that long term it's too risky not to heal in such situations.
Mass cure wounds cast by a Life cleric is a great example: If you do it when the entire party has taken some heavy hits, you have a good chance of purchasing 2-3 rounds' worth of actions (one for each teammate who is saved from eating dirt for a round). Or suppose the party tank has a stratospheric AC, such that they rarely get hit. In that case, a big healing spell could purchase 2-3 rounds for that one character. The results are not as certain as whack-a-mole, but the potential upside is much greater.
Sure. The worst case scenario if you heal in this situation is that you've used a spell slot and restored hp that your allies don't actually need to win this fight. They still keep those HP's etc. The only real downside is you risk being in a situation later that day where you end up needing that slot and you don't have it.
And if the DM is playing monsters "viciously," so that they go hard after downed PCs, the potential upside is an entire adventuring day's worth of actions (or however long it would take a slain PC to be resurrected or replaced). However, this is a case where you are spending an action this combat to buy actions in future combats, so you would only do it if you were fairly confident of winning the current fight - if you're on the ropes and facing TPK, then future combats are irrelevant, the focus must be on surviving this one.
Spending your strongest action this combat as I advocate for would fit perfectly in this scenario. It literally would be you doing everything you can to survive this fight without thought of future combats.
I more or less agree with you: In-combat healing can be a useful tactic. It's just a matter of figuring out how to get the best value from it and knowing what spell to use when. That depends on your party composition, the adventure, and the DM.
I agree. I just think you have to look at pros and cons both when evaluating risk. So for my tactic, which while on average there's often a tiny amount of downside, there are times it has immense upside. Compared to whack-a-mole healing which while it often has a small upside, there are times it has an immense downside.
Generally speaking when managing risk for something extremely important you want to minimize extreme downsides even if it costs a little more overall. That's the foundation the insurance industry is built upon.