I agree with you but while it would be nicely convenient for posters, it probably isn't as great for the promoters. If you can make good posts, but you're more casual, and another user makes posts of equal or maybe even less quality but posts a lot more than you, I'm pretty sure the latter is more attractive from a promoter standpoint. So while we can only wish there are more lenient campaigns like this one, there probably won't, hell even for this one, I'm sure if you post below (a hidden minimum) quota for long enough you'd probably lose your spot..
From what I've seen, spooky minimum quota is 3 (I'm not entirely sure on this one though, I could be completely wrong and it probably depends on what those three are, if one of them is to say you want to halt all trades you've been doing recently due to personal circumstances, darkstar_ goes through everyone's post history enough to see it and probably allow some leniancey on the removal of your place here).
Someone had about 3 posts every week for a month I think before darkstar kicked them. There's not much point in you taking up someone else's slot if you're not going to make that many posts here.
I think the average is probably somewhere between 10-30 (I haven't checked the spreadsheet recently so can't be sure).
If you're a legendary who has been here for a while, that means there's at least 750 posts out there with your signature on them, one of thos posts is probably most likely in a topic where it will be seen by other people (a lot of guests will search stuff and come accross certain things, such as, if someone like me stopped posting in the technical support boards and someone did a google search for the forum for that specific issue they'd then see my thread and see the "chipmixer" signature). Similarly, someone like TheQuin would have his signature in the freebitco.in thread quite a lot so someone going "hmmm, lets try some gambling but the casino is illegal in my country and I did kc on an exchange to get the money" will see thequin's signature and think "aha! I'll use that to obfuscate my transactions" - a lot of people use bitcoin mixers, some people in trading probably use it to hide a strategy they have that's quite profitible for example so the exchange won't block it (or shouldn't) because they're stil getting their fee...
With the issue of there being 59 spaces (and not 60), this i due to the original campaign wanting to take 55 people. There were then 4 people of greater trust that got accepted here (or a bit of darkstar indecision between some great posters) so the spots were extended to beyond maximum capacity.