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.
Unprotecting questions is rare (but it shouldn't be)
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.
Protections are carried out by only a handful of users
This whole discussion was kicked off by one user who unprotected 99% of our questions. However, it should be noted that ~ 50% of those protections were carried out by a single user as well.
~50% of all non-automatic protections on this site have been carried out by 1 user.
To be clear, there's nothing wrong with that user doing the protections. And there is a clear difference between the two cases because this user did these protections over the course of years as opposed to the unprotections which were done over the course of a couple hours.
But I bring it up because I think it shows how little transparency there is around this whole mechanic. The vast majority of users never even interact with it at all (except an unknowable number of 1-rep users and spammers who get blocked from answering).