Yep, this is an annoying weight weenie-ism. I just weighed an aluminium body and an oldish 9sp steel Shimano one, and the difference is a measly 43g!
The cogs can eventually be very hard to get off, which is obviously a drag if ya wanna change wheels regularly.
Campag is a better design for this. Unfortunately, Shimano designed their bodies way before alu was common for this use, so it's not a good design for a soft metal.
Also, the threads for the lockring are a lot easier to strip.