Pure aluminum is incredibly light but also quite soft and expensive. Aluminum alloys are tougher but still expensive. Most budget cast items are made from an alloy of magnesium, zinc, and aluminum. THe magnesium and zinc do the heavy lifting of being light and cheap. There will also usually be a silicon component used as a sort of filler and sometimes a zinc component used to aid flow into the casting.
If the casting is very cheaply done, it may just be an aluminum silicon magnesium mix. Higher strength alloys such as the 6XXX and 7XXX alloys will have added materials increasing the overall density such as iron, copper, zinc, manganese, and titanium (as in 6061) which makes our social perception of 'aluminum' seem heavier than it really is. Cheap castings will also have a much lower density than a homolgenized, graded alloy due to crystalline porosity, lending to a reduced weight. parts made in this way tend to be very stiff but ultimately quite brittle and significantly weaker than their constituent parts. If you have ever seen something made of a light metal with a fracture that looks very 'sandy' it is usually the result of a porous casting caused by poor process control or hastened cooling. A well-made part in cast aluminum should have a break that seems to 'smear'.
Bennet's insurance in the UK has some great videos on bike locks which show what it takes to dissuade a bike thief. The torx bolt is much less for security and more to dissuade the curious and allow establishment of a torque spec IMO so that a manufacturer can have some recourse for 'not following spec' even if the original design is subpar. Just my hot take. If you have some sort of adjustable doo-dad under the seat then a finger bolt makes sense.
I'd just worry about galvanic corrosion:
alu + brass/copper + conductive fluid a.k.a. water = electron exchange = corrosion which is avoided with nickel or SS
Which might self-lock the threads; which then stands the risk of breaking the plastic mold bond around the threaded insert while the seat is on the bike which might become a safety hazard or prevent execution of work such as changing the battery with might result in a tow.
Long story short, I think it;s all still personal pref but I think the two points above are why BMW chose a SS/Ni-washed fastener for the brass alloy washed/plated/metal threads in the seat. Still unsure why it isn't just a half-depth spring-loaded 60* throw key lock but that's a question we may never have an answer to.
as I’ve had mine (2) for 16 months now and yet to see another when out and about!
