Spreading in UMTS always puzzled me as it seem to add redundancy and I thought its good for data recovery but bad for capacity. Now having some more experience here is how I think it works. Please correct me if I am wrong (example case of voice call):
Spreading -> Adds Robustness -> Lower power consumed for to establish call with acceptable quality -> More power available for other calls
So the more power available is actually the increased capacity we expect from 3G!