I am looking at 0% Alta crank pulley and I am wondering why car manufacturers install heavier pulleys.. i mean, i heard 0% is equivalent of shaving 100 pounds off the car. why doesn't it come with lighter pulley off the factory? it's not like i'm jeopardizing reliability since it's the exact same size.

 

There are a plethora of threads on this topic.
I'm not the most technical in my explanations, but this is just a summary to entice you to search.

The stock crank has a layer of rubber between the belt track and the shaft to dampen the engine's resonance. In the long run, this resonance, if not dampened, could cause engine failure - some experts believe.

I returned the 0% crank I had ordered a year or two ago because I was fearful of resonance damage to the engine. I plan on keeping this car forever.

Yet, there are some that claim that the belt itself would help dampen the resonance. They compaired it to putting your hand on a ringing bell to stop it. According to them the lightened pulley would be ok. This would be true as long as the belt does in fact provides the same resonance suppression as the stock part did.

 

Bear in mind that the 05 and 06 MCS had a lighter pulley than the earlier cars so your results may vary.
The question of long term engine durability is a valid one, with a lot of debate. IMO there's a reason every major car manufacturer uses a harmonic balancer rather than just a standard pulley. Until I actually think I'm smarter than every automotive engineer (rather than just pretending I am) I'm sticking with the stock damper on my 06 MCS.
Study carefully and think critically before making a final decision.