I thought the main benefit to aluminium bodies was reduced wear ?
Absolutely, the main benefit once you have a set is reduced wear but there's more to it.
I have no direct proof of any of this, just unofficial info gained talking with the Amal staff as a dealer...
As I understand it, the initial reason for the change is Amal's inability to find a foundry able to make the components reliably from the old alloy - the reject rate was VERY high. For several years they could not keep up with orders for that reason. They had already started making some 600-series carbs in aluminum and were having no trouble getting quality components. Their costs were going up and they kept having price increases all while thinking no one would buy. Also, along the way they had developed things like anodized slides and "StayUp" floats and they well knew of the "bush" pilot jet problem, and it was obviously a simple change to fix the pilot circuit.
So, Burlen could either continue making Amal carbs or stop and unless they did something, stop was the only answer. I had unfilled orders for over a year and getting 932 carbs was just not happening. 930s were somewhat available because there was a much bigger market. Today, I can order most Premier sets and get them quickly.
The three really good side benefits are that aluminum bodies and anodized slides wear much less, the machining in better, and the carbs weigh quite a bit less.
Although Amal still lists non-Premier carbs ("ACK" sets), they won't be available again IMHO. I'm hoping that much better foundry reject rates and focusing on Premier ("PACK" sets) will allow them to quit raising prices.