I think there's a specific, and very clear, problem with telling Harry: Harry of all people knows the Noose makes Nicodemus almost invulnerable, but not precisely. The Spear of Longinus also makes one (all but) invincible. If Harry knew that Nicodemus was after the means to make himself two-layered invulnerable, he probably would have blown everything up.
Instead, as he discussed about the Grail with Michael, he doesn't precisely understand what Nicodemus could possibly do with it, so he was a bit more sanguine about gambling on it.
It's interesting, here, that the 'gospel of Nicodemus' is the first that goes into detail on the whole Longinus angle.
The myths surrounding the Lance of Longinus (assuming that's what it is, and I've stated previously that the description is a very close approximation of how you would describe hastae) don't indicate that they make you invulnerable, but unbeatable, a slight but significant distinction. The Spear of Destiny doesn't have the same effect as the Noose. The myths surrounding it—that Charlemagne slept within reach of it, and credited it for something like fifty military victories, or that Hitler seized it in the 1938
Anschluss, when Germany annexed Austria; the legend goes that he went straight to Vienna and took possession of it after seeing it as a young man and feeling the "pull of destiny." Basically, the legend goes that as long as Hitler held it, he wouldn't lose, and that when Patton seized control of "several religious artifacts," Hitler lost, and promptly killed himself. The legends
also say that losing possession of the Spear means you die, and that Patton had shipped out the religious artifacts back to the States, and was promptly killed in a car accident on base. (This is based on spotty accounts that have been sensationalized greatly over the past sixty years; there's really no evidence that Hitler had it. A man named Trevor Ravenscroft wrote a book about Hitler and the Spear in the seventies as a way to explain how a talentless Austrian painter could have become one of histories greatest monsters. His source was "a friend," which should tell you everything you need to know.)
Basically, the myths indicate that the Lance of Longinus make you a badass conqueror as long as you have it, but the second you lose it, you pay the price. The whole invulnerability thing was popularized by movies like Constantine (which is still a favorite of mine—I think it's urban fantasy done right, and couldn't have been more disappointed in the show).
Anyway, something that fits the myths surrounding the Spear of Destiny would be invaluable to Nic's long-term plans; they'd all work out how he intended. (Also worth noting that the moment Dresden picks it up, all of his plans work out extraordinarily well.) Speaking of which, what happens to the "leaf-shaped blade" during the fight at the Carpenters'? Does Butters take it when he picks up Harry's duster? Cuz that would explain some things...
