They're mostly there to be a reasonable budget choice; they're entertaining but not actually that reliable/good. All other things being equal (with neither side charging and not counting Legion tactics) in an artificer-armour sergeant match the one with the powerfist wins 2/3rds of the time, and he's got the ability to punch vehicles/
ID multi-wound T4 models/engage monsters effectively, which the saber doesn't. I'd give a character a power weapon/power fist over the Charnabal saber every time, though depending on your meta it could find a place on sergeants. And on an Auxilia Marshal or Militia Commander who's already kind of screwed if he's in melee and who gets the saber for 5pts it might be funny, occasionally.
If you like the idea of the Charnabal sabre the Emperor's Children, Death Guard, Night Lords, Raven Guard, and White Scars all have access to better AP2 or Rending weapons on any character that strike at Initiative (though they are usually 15-20pts rather than 10pts), and the Blood Angels and Ultramarines have unique AP2-at-Initiative weapons available to all Independent Characters but not to Sergeants.
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The only way I know of to do this is Palatine Blades, and even then the option to upgrade to Phoenix power spears means that for the low, low price of +50% to the cost of the squad (+33% if you've already given them jump packs) you can double their effectiveness against T4/2+ units.