Spam prevention doesn't seem to be the main use for protection (volume-wise)
Given the previous data, this would mean that only 212/943 = 22% of protection activity involves spam.
58% of posts which receive spam are never protected
This tells me that spam has been not the main use of protection for us in the past. In fact, it seems like a very minor or at least inconsistent use for it. Since we've only ever had 79 automatically protected questions with spam on it, this also tells me that this 58% of unprotected spam doesn't seem to be attracting a huge volume of repeat offenders. (If there was more than one deleted spam post, it would get automatically protected).
It is possible that we are just very good at picking out which spam posts will draw more spam and that could account for some of the low usage (and maybe even some of the 31 questions that got unprotected, even with spam on it).
This doesn't mean that we shouldn't protect spammy posts of course, but we should be aware that at our current very low usage of protection for spam, we still only have around 4 spammy answers a month. A very manageable amount.
We are bad at unprotecting questions
To quote doppelgreener's meta answer here,
As BESW summed up in chat, questions tend to get protected for one of three reasons: (1) a question hits HNQ and attracts nonsense, (2) a question is controversial and needs a bit of brakes applied, and (3) a spam algorithm has identified that question as a target via keywords.
Assuming this is true (and both BESW and doppelgreener are certainly experienced enough users to have developed a feel for this), somewhere around 2/3 of our main uses of protection are explicitly temporary. HNQ only lasts 72 hours maximum and thus protection for this reason need not exceed that amount of time. Controversial questions are often the same, when the question becomes less active, the need for protection fades quickly away since the volume of posts slows to a trickle or stops completely.
However:
Of all the protected questions, 149 questions, (15%) ever got unprotected
Ideally this number should be higher following from the argument above. But that's not really entirely our fault. There is no timed or conditional modes for protection; it stays until it is sought out and removed by a user with enough rep to do so. Without any kind of notification or prompting to unprotect, it is unsurprising that it rarely gets done. Especially since the users that have the rep to do so are entirely unaffected by the protection.
We do need to remember that protection completely shuts out the potential for a new user to answer that question unless they receive an upvote somewhere else on the site first. We should be reducing the times this tool is used unnecessarily to reduce the barriers for new users.
Only 2% of HNQs get protected
Since Feb. 28, 2019 (which is when the HNQ status started being tracked), 51 questions have hit HNQ and also been protected at some point. This represents ~2% of the 3052 HNQs we’ve had over that same period of time.
So, it was surprising to me that only 51 questions since 02/2019 have hit HNQ and been protected given that HNQ is often cited as a big reason questions are protected. It doesn't necessarily seem that that is the case however or at least it isn't a huge driver number-wise.