One of the bigger wanks in our sport is people fitting alloy swingarms thinking it's going to transform their bike into great handlers. They're purely a bling thing, nothing more. I've got an alloy B&S swingarm on my squarie but I'm honest enough to admit it's only there for show/bragging rights. If anything, it's less rigid than the stocker.
I remember reading somewhere where Maico works rider Herbert Schmitz was racing in America aboard a 490 Maico fitted with a Thor swingarm. After only one race he told the team to replace it with a stock Maico item. Turned out the stock arm was way lighter and noticably more rigid.
It must have been that fancy chro-Mo they were made from. 
Rigidity/stiffness etc comes from design NOT the material. The stock arm could have well flexed more under load and given better feedback, which requires a works rider to pick?
