if you're asking whether or not you should bother glyph'ing BS as unholy in 3.2.2, then I think the answer is unequivocally yes, you should. I think blizzard assumes you'll have the glyph while tanking, because 3 charges isn't going to get you much in the way of additional mitigation unless you're using it to ward off a single attack, like a Fusion Punch. I also don't see it getting you through 15 seconds worth of Meteor Fists on Koralon. as for your other questions, I can't think of any fights where spike damage, in the way that it's discussed and mentioned on this forum, lasts longer than 15 seconds. Fusion punch is more or less a one-shot deal when it occurs; Koralon's meteor fist was already mentioned, which is exactly 15 seconds in duration. If a spike damage moment is lasting longer than 15 seconds, I doubt blizzard would balance it around the idea that you're using a mitigation cooldown to get through it.
1) Are there any forms of spike damage that last longer than 15 seconds?
2) TOC, with it's lack of spike damage and plethora of physical damage would probably be better tanked as frost (or blood). If not frost then as Satorri mentioned you would use BS everytime it's up. Ulduar (cooldown style tanking) might not require more than 3 bones
3) What is the threat tradeoff? (A few more seconds of 20% extra damage, vs glyph of unholy blight)
4) Does the question even matter? With the RNG style of BS an extra bone could just be "mandatory" for aspiring UH tanks.