Yeah bro, if you have to dedicate all 3 slots for mana passives, go on right ahead. However, if you can afford to take 1 or 2 slots for dmg/survivability passives, the better!
Don't get me wrong. I don't think Manajuma knife is a bad weapon. In fact I think it's one of the best weapons out there for WD's. It's cheap, strong, and most especially, it rolls 4 guaranteed stats that I absolutely love and need: LS, Int, Crit dmg, and %weapon dmg. But if you were able to find a black weapon with the same dmg and stats, using the black weapon would do you more good. My preference in what stats should roll on a black weapon (in order of most important): atleast 650-700 ave. damage, high LS/LoH or both if possible, %weapon damage, Crit damage (if weap doesnt roll CC, open socket), Int. If you reach a point where I can afford to lose one of those, I go for Mana regen. However, black weapons with those stats are very rare in the AH. You can barely find one with all the stats you want and when you do, it will cost you both of your legs or even more!! lol.
As you can see, I have an EF equipped. It's a good black weapon that can go as high as 1.3kDPS (or higher. not sure). The only thing I hate about it is that it rolls fear as a guaranteed stat and sucks whenever you have fast/illusionist/teleporter ellites and they just run like they're in a marathon or something . But other than that, I found it a great replacement for my mana knife because it has double the minimum damage of the mana knife and basically, MOAR DPS MORE POWAH!!!
If you actually look at and compare our stats, you have 150 more vit points than me. I still have a few crappy gears on as I don't play much so yeah whatevs. lol. Anyway, If you can get your hp to go as high as 70k or even past 100khp, why not right? But to me, I tend to think that anyone can get away with even 30k hp (anywhere lower is just plain stupid) so long as they balance it out with armor/ AR/ LS/ LoH/ LifeperSec. Right now my EHP is 380k which looks average. But honestly, I am nowhere near balanced as I have too much AR, not enough armor, no LS, average LoH but could be higher with BS jousting pants, low lifepersec (without BR), and to top it all off, way too low DPS (because technically, if you have 10000000000000 DPS you can kill everything faster. When you can kill everything faster than fast you, you get hit a lot less often. And you don't get hit as much as before, you can get away with anything! technically. lol) It's all about making up for whatever attribute you have low. Juggling. Or if you can increase everything all at the same time, why the hell not!!! lol
PS: Terribly sorry for the long reads. I can be too wordy. I like elaborating. A LOT. lol