You are only looking at one aspect of what a yeast starter is all about. The first reason for a starter is to confirm viability, you only have to read this forum during summer how many people have cooked smack packs.
The second reason for a starter is to improve yeast health, yeast consume their energy reserves while dormant so when they come out of your smack pack they need to build their energy reserves. A thick wort is not the best yeast food, the high levels of sugar in the wort increases the pressure on those weak cell walls.
The third reason is to boost numbers well you say pitch more packs the problem with this is you don't know how many viable cells you had without the starter, is the pack old was it handled well between producer and you...
So, long and the short is do the starter you will have better beer and a quicker cleaner fermentation.
Clem