Not worth it, IMO. As others have said, you don't need it by the time you get it. Also consider the perk cost: Stealth 1 + Muffled Movement + Light Foot + Silent Roll + Silence + Shadow Warrior = 6 perks, and most of them are worthless. Generic enchanted items and/or Illusion magic will be vastly superior.
I have to differ with your assessment. First of all, you can't even count Stealth 1, since it's also a pre-requisite for the excellent backstab line, not to mention the first point is a whopping 20% boost to sneak (which is applied to your fortify sneak potions/enchantments). Second, the intermediate perks are far from useless to a stealthy character:
Muffled Movement - Fantastic early on, when you may not have access to the late-game heavy-hitters such as muffle/fortify sneak enchant, conditioning, unhindered. I don't know about you, but my sneak skill rose considerably faster than light armour, since I made it my business not to be repeatedly punched in the face. Later, yes, it's redundant if you take one of the former options, but the thing about getting to level 50 is that you have to get through level 12 to get there.
Light foot - This might be my favourite perk in the entire game. Yes, traps aren't that devastating in Skyrim, I wish they carried more punch. That still doesn't mean that being able to ignore every trap except tripwires isn't incredibly luxurious.
Silent Roll - This is an ASTOUNDING perk if you're good with your controller. I've rolled right past dungeon bosses to set up my backstab on them, and been able to get to places undetected that I could never reach by conventional tip-toe.
Silence - This perk is the bomb. Getting the same stealth from running as walking? Again, this makes it far, far easier to into position, avoid detection and otherwise run rings around the enemies trying to find out.
Now, you may argue that you could just as easily throw on +120 fortify sneak on your gear and never be seen, but I put it to you that having the above perks free up your enchant slots for stuff that's actually useful, like magic resist, fortify health and stamina, and stamina regeneration, fortify magica, etc. And all the fortify sneak on the planet wouldn't get you a re-stealth on an enemy
right in front of you.
And lastly, what on earth else are you going to spend them on? By level 50, you can easily have enough perks to buy enough enchanting, smithing, alchemy, weapon and armor perks to moonwalk through the entire game, with plenty to spare.
You may feel that Illusion may be vastly superior, but it's not in concept for every character, and even if it were, you're still discounting that illusion costs mana (unless you invest in 3-4 items with fortify illusion), and sneak perks do not. Illusion doesn't worth on all enemies without deep perk investment itself.
What I do know is that I did the battle of Whiterun on my assassin, and was able to slit the throat of nearly every enemy I ran into. At that point, what else do I need to justify the investment of a handful of perks? So I can dump a bunch of enchantments and perks into a different tree and do the same thing, just not as fast?
I'm pretty sure you get 15x on both weapons as long as they are both daggers... and I think that damage is applied as one attack. You'd have to check me on that one, though. I like Valdr's Lucky Dagger in my right hand and Mehrune's in my left. Both can be smithed up to around 90 dmg without exploits. 2(90x15)=2700=kickass
Yes, a double-swing calculates sneak attack damage from both weapons. However, I MUST recommend trying wearing the Dark Brotherhood gloves and getting the x30 damage backstabs, because the throat-slitting animation is
unbelievably satisfying. I'll get my act together later and post my ultimate assassin build. It is exploit-free, totally viable from the beginning of the game to the end, and the results are 'Invincible, Invulnerable, Invisible'.