I am quite new to rigging. What I saw - are few cases, when slinks on reserve were not sewn. Well, probably it's more question of "religion", but I believe, that if manufacturer says "it must be sewn", then it must be sewn.
Another case: Month ago I've got for assembly one rig with very old container, where the chest belt was replaced. The replacement was done with wrong thread and some strange sewing pattern. The container was not packed before and I refused to pack it as well. That's why it can't be considered as rigger error... at least at that time, when I got it.