Doublesheath with a ninja isn't one of the most widely taken selection choices, most people going the assassination route (falcon eye, that other one that I can't remember) or the more melee-geared route (lotus scourge, shadow arts). Doublesheath for a ninja allows the ninja to throw a dagger when opening combat with murder and when the opponent flees. Useful, yes - but I'd sooner take one of the other paths.
As it stands now, the Ninja cannot actually use doublesheath to doublesheath a weapon. At least, the ninja is told they can only doublesheath exotics, yet with mine I couldn't even doublesheath those.
How about allowing a Ninja to use doublesheath in the way a warrior would? It's not uncommon to see melee ninjas (ninjas? ninja? ninjae? what is it?) using both two-handed staffs and dual-wielding weapons and allowing them to swap in combat as well as having the disarm safety barrier would make it more of a useful, viable selection - without overpowering the ninja, IMO.
I've played a doublesheath ninja and was quite disappointed when I found out I couldn't do the above. It's not 'underpowered', but not as appealing as the other routes currently I think. The thread in newbie forum sparked it up in my mind again.
Thoughts?
Dey
Well, thieves can do this.
But then again, I've noticed thieves don't have CLOSE to the throw damage of a ninja.
BUT....if you take double sheath, you can double sheath or play hit and run with your opponent until you make them run and get your auto throw on. You go, Groovy Dude!
murder dey
flee
Dey walks in.
murder dey
flee
rem weapon
draw
murder dey
Dey flees!
You throw shurikens at your opponent as he flees!
You throw OBLITERATES Dey!
Obliterates?!?! Not without vuln & no sanc and an ounce of luck. Thieves and ninja/ninjae/ninjas/ninjii/ninju/ninjai/whatever had their throw damage toned, it's bards that are the heavy hitters with throw.
Ninja shurikens don't wound like thief daggers do either, so no blood-loss. Envenomed shurikens/daggers are a nice bonus, but I still (personally) think doublesheath is lacking when compared to the other selectables. Allowing them to change weaponry in battle wouldn't be overpowering, and would probably be mostly used when fighting melees - which the ninja already has to think outside the box when fighting. It gives the melee ninja the oomph - switching from pugiled staff to dual-weapons to finish them off quickly, or switching from dual-wielded weapons to staff when caught off-guard.
Disarm protection (again, useful mostly when fighting melees) that comes with having a doublesheathed weapon gives them a little more of a fighting chance when disarmed against Mr. Decked Warrior or the like.
Just for the record, not saying the melee-style ninja is underpowered at all - just that I think I'd sooner take Lotus Scourge or Shadow Arts. Or, perhaps I should just sit down and shut up. 
Dey
I think I've hit an oblit on an unsanced oponnent last year...could be wrong though. But bards....bards hit much harder with their cards...damn paper cuts...
shrug I'm not sure what to say about this. Balance issues and numbers aren't really my forte, so don't bother sitting down and shutting up....I've done enough of that in my 7 1/2 years here for both of us. 
Hmm, I might have to toy about with one and see what the throw is like. I can't remember for sure, but I seem to recall MUTILATES to MANGLES as being the norm, usually around a DISMEMBER. Sanc or no sanc, I don't remember. I know there's a log somewhere too, I'll have a look at it - Hankith was a double-sheath ninja.
Hitting a vuln on an unsanced foe could easily produce an OBLIT I think.
Dey
I don't know about that man, every ninja I fight seems to have doublesheath. Either that or falcon eye (I assume). I definitely don't think it's underpowered, even though I said it was overrated in the other thread..it's just fine the way it is.
Ah, hmm. Fair enough.
Like I said, I should just sit down and shut up.
Dey
The plural of ninja is ninja. It can also be ninjas, but that is more rarely used.
Thieves only throw 1 dagger, while ninja(s) throw (I think) up to 5 shurikens, and bards throw up to 6 cards.
Why do shurikens have holes in the middle of them?
So if you accidentally swallow one, you can still breathe.
The fact that ninja's can't even double sheath daggers is what bugs me, Your stuck with trying to sort your weaponry according to the exotics, when there are better weapons you were rather be using.
Ninja's now have to choose double sheath - I would imagaine - due to balance reasons. If you give it out all ninja can auto/backcutter throw without giving up any other skills.
This again allows the builds of melee/falcon/death touch that gives ninja the best of all worlds. I believe it was changed to how it is now so you have to choose and can't have it all.
The only real downside I have seen is that its hard to get mileage out of lotus scourge (IMHO) without the autothrows. No idea how to remedy it, but its probably a minor issue since almost all ninja almost always take falcon eye or double sheath.
L-A
It is pretty strange that ninjas can't doublesheath daggers, considering they need something pointy to assassinate (I think). I think they should be able to doublesheath daggers and only daggers.
Obviously other factors are involved, but I mean...they can't assassinate with a staff ya feel me? I could see them finding somewhere for a small dagger amongst the poison and blindness dust packets.
Ninjas can assassinate with any weapon, last I played one.
Well, any weapon I think, but I've never tried with a weapon I'm not proficient in since I'm pretty sure your chances are miserable that way.