Needle weapons have always been hideously rare and costly, but the problem is that they're underpowered for how much effort you need to get it. You need to buy an exotic weapon talent just to use it, they have small magazines, and finding extra ammunition is difficult at best. All told, it's much more cost-effective to just buy a sniper rifle and slap a silencer on it, and then load up with whatever specialty ammunition you want (which comes in far greater variety and quantities and is much easier to find). At the distance you're typically going to be shooting your targets, they're not going to be able to hear a silenced gunshot. The long-las is also a very viable choice, since you're unlikely to ever run out of ammunition, and its Felling (4) and variable power settings (and the use of Hotshot packs) make it much more powerful than the anemic needle rifle, even with the Whisper-Bolt Discharger (from Rogue Trader: Hostile Acquisitions; a lasgun "silencer" that reduces damage by 1).
For the cost of a needle rifle, you could buy a Solo boltgun (from Rogue Trader: Into the Storm), a far more powerful weapon with a bigger magazine, and load it with Stalker silenced shells. There are plenty of alternative sniper weapons you can get, and they all outperform the needle rifle in pretty much every practical way. The needle pistol is even more useless; last time I tried to use one while playing Rogue Trader, I ended up doing more damage with a laspistol. When the Imperial Guard's mass-produced flashlight is more powerful than your expensive, exquisite assassin's weapon, get a better gun. They're just not worth it.
Edited by Boss Gitsmasha, 23 January 2014 - 01:56 PM.
"Oomans are pink an' soft, not tough an' green like da Boyz. Dey'z all da same size too, so dey'z always arguin' about who's in charge, 'cos dere's no way o' tellin' c'ept fer badges an' ooniforms an' fings. When one o' dem wants ta lord it over da uvvers, 'e says 'I'm very speshul so'z you gotta worship me', or 'I know summink wot you lot don't know, so yer better lissen good'. Da funny fing is, 'arf of 'em believe it an' da uvver 'arf don't, so 'e has ta hit 'em all anyway or run fer it. Wot a lot o' mukkin' about if yer asks me. An' while dey'z all arguin' wiv each uvver over who's da boss, da Orks can clobber da lot."