This might be helpful for your specific question about chickpeas: caloriecount.about.com/forums/foods/chickp
It gives gram measurements but that's easy enough to convert to ounces.
Sometimes when I'm trying to figure stuff out, I'll just google my question. So like for your chickpeas I googled "how much is a half cup of chickpeas in ounces".
The only advantage of using cup measurements is ease of use. The normal cook, at least in the US, does not own a kitchen scale. So, things are written with volume measurements and everyone just uses a set of measuring cups to measure things out. I personally prefer weights too.