This sounds good until you realise some gangs have 1 or 2 skill sets that they can viably use without taking a garbage skill. Also a hypothetical for you, how do you handle a delaque gang with 4 psykers at creation. They are locked to 3 disciplines at creation
Agreed - some skills are not optimal ("garbage skill"), that is a facet of the game we are trying to foster - fun, narrative, diverse, interesting.
Does that mean some gangs will definitely end up with sub-optimal skills? Sure. Arguably, some have pretty garbage skill options from the get-go (*cough*Cawdor Champions*cough*)
Does a sub-optimal skill in a gang of 6 - 9 dudes "ruin" your competitive edge? If that's the win-at-all-costs razors edge you are playing by, Necromunda (or at least, Necromunda in our play-group) is perhaps not the game for you - something more competitive, with a tested, cohesive and balanced rule-set, might be more up your alley.
Similarly, your example of a Delaque gang with 4 psykers... okay, Delaque psyker skills are pretty trash so I don't think this is ever going to actually come up, but I'm sure the arbitrator could make a call here if someone
was in such a bind - probably allowing a duplicate discipline in the mix but with a unique power.
This is a game played between friends at the end of the day, for fun and laughs and ridiculous stories of that time your Juve's molotov scattered hard and set your own champion on fire who then fell off a gantry and fell to his doom in a ball of flame - we're playing with beer and pretzels, not running a tournament
