Aluminum does strengthen the once weak spot, but it spreads the stress out to other areas. Aluminum also adds weight to the vehicle, so throwing more than a few aluminum parts on it will drop the performance (power) a little bit since the engine will have to push the extra weight.

Aluminum bends, plastic flexes. Plastic is cheaper to replace, but breaks easier. When aluminum breaks, it can cost about 3-4 times more to replace it rather than replacing with a stock plastic part.