The DPS and skillchain opportunities granted by primes are substantial
I can't get too excited over the new skillchain options. Sure, a 3 step darkness chain with the same high damage WS in all 3 steps sounds promising but we are in the WS wall era, imho a shot in the foot of the prime weapon concept.
They aren't only "3 step by itself" weapons though. They are also unique in their weapon type, with very few exceptions and no good exceptions.
Dagger: Only other Fragmentation WS are Mordant, Shark Bite, and Exenterator. Some jobs can't use any of these, and most are between bad and terrible.
Sword: Only other Distortion WS are Expiacion, Death Blossom, and CDC. These are mostly terrible and/or job-specific
Polearm: For Gravitation, Stardiver is not great and Impulse can't hold a candle to Diarmuid
Club: There are exactly 0 Gravitation Club WS
Katana: The only other Fusion WS is Blade: Shun which sucks donkey nuts
GKT: Less important SC property (Distortion) but it's still the highest-damaging Disto WS and can set up Gravitation (in 2-step) better than anything else by far (Rana sucks)
Archery: There are exactly 0 Gravitation Archery WS
Marksmanship: Has other Fusion options, but it is the highest-damaging Fusion. Also the only realistic option for COR to bring Frag (2-step, like SAM)
H2H: There are exactly 0 Distortion H2H WS
Staff: Not really used for this, but for non-SMN, it's the only Fusion Staff WS
GS: Not all jobs can use Torcleaver, Ground Strike sucks, it's the best Distortion option by a mile for WAR, RUN.
Axe: The only other Fragmentation WS is Cloudsplitter, a magical WS.
Gaxe: There are exactly 0 Gravitation Gaxe WS
Scythe: Insurgency is the only other Fusion WS and is FAR outclassed by Origin.
More impactful for some than others, but it's really not as simple as "do a 3-step by yourself" in terms of opening up WS options for these weapon types.