As far as I understand it, re-steeming is the ultimate upvote. Maybe you get a percentage of curation rewards from upvotes that stem from your re-steem. But it should only be done when you really want your followers to see the particular post, because you think it's brilliant or exceptional in another way.

But I've unfollowed people for re-steeming nearly every trending post, because their blogs became unreadable.

For now, my own policy is to never re-steem, I've got a certain style to my blog, and I don't want to disrupt that. I hope that one day my blogs become so good, that after reading one, you'll want to read the next...