Does anyone have any good reason why I should bother seeking out this rare armor?

It statistically doesn't look much better than Maximillian/Shishak...

If you're the type of player who wants to get as much of the cool stuff as you can, then go for it, but if your characters are strong and well-leveled, you can probably get by with a g-string and a viking hat. (no, I don't know where to find them) :)

So where exactly do you get this "Grand," but apparently only so-so armor?

Helvinek drops it. Rarely. And Helvinek only appears in the Necrohol. For a rare mark, he does show up pretty regularly, but all I've managed to get from him is a magic lamp.

same here....i've killed that Helvinek like 3 times and it hasn't dropped any yet. but it seems to appear when you clear a certain area of all monsters..