I haven't got the dagger/cloak yet but assuming max stacks you can get +5 multicrit from them. Running shadow as carry when reaching end game I use excalibur, blade of darkness, seelenreisser and demon blade. Demon blade and seelenreisser are no brainers. Excalibur + blade also give +5 multicrit so basically the dagger/cloak is a little worse since it needs to stack up. I'm disregarding the armor pen it has because you obviously have a knusper completley mitigating creeps armor anyway.
So not so legendary...
I suppose you can get +1 more multicrit if you're using a carry with a very high base damage so that it gives more total damage than using a demon blade.

I don't know, Base Damage is pretty frigging swell. Honestly, I'd probably hand off Seelenreiser if I got the set. Not only do you get +1 Multicrit back, you can also boost the living hell out of a tower not using multicrit already. If you're talking dedicated Darkness, I'd likely run your carry with Excaliber, Blade of Darkness and the new pair. Would give you a stupid amount of Multicrit. I'd likely pass off Seelenreiser to the Dark Forge.
Note, the Demon Blade for me isn't all that fantastic, as I run a Ripper carry, not Shadow. If I were using Shadow, I might look at switching out the Blade of Darkness for the Demon Blade.

My big problem is the limited scope of utility.
It is ONLY useful when:
a) You are hitting a single target (i.e. challenge)
b) You have tons of crit CHANCE so you can take advantage of going from +6 to +11 in the first place. (i.e. >150% chance)
c) Your tower hits really fast.
Therefore, the only time these "legendary" items are useful are when doing round 200+ challenges and horsemen!
Even then you have to hit quite a few times just to GAIN any multicrit, which only applies as long as you hit the same target. At max 12% per hit, plus luck bonuses, you have to hit 20 times to max it. With a Frozen heart you'd have gotten 20 extra multicrits out of it.

Is it necessary to boost past 100% Critical Chance? And while I agree that the Spectral Set isn't game-endingly great, but it still looks pretty amazing to me (I don't have it yet, full-disclosure).

Well, i can't give you any exact numbers here but:
but if you reach over a certain point (lets jsut assume 100%) the chance to get a higher multi crit goes up.
Lets ay on 100 % Critchance you have
60% Chance for 1 MC
25% Chance for 2 MC
10% Chance for 3 MC
5% split up to the MC 4-8
If you have 150% Critchance its like
50% for 1 MC
20 for 2 MC
15 for 3 MC
10 for 4 MC
5 for 4-8 MC
So if you have more then 100% critchance, you increase the chance to use a higher MC.
Like i said, these numbers to not represent actual values. These are just random number to make my point =)

Wow i just got how MC work
As i calculated
to get 100%chance of full MC
you need 1.25^(MC) * 100% chance
so 2MC > 125%, 3MC > 156.4%, 4MC > 195.4%, 5MC > 244.2%, 6MC > 305.2%.....

Wow, I always just eyeballed the damage increases and assumed they were all scaled based upon the 0-100% Critical Chance (As is, at 100% chance, you would guarantee your max amount of criticals).
The more you know!