I pay the balance every month, I wouldn't do it otherwise. Think of it this way, if you spend 3-10k a month in everything that you buy, that is basically a round trip flight every 2 to 6 months depending on where you go for just spending money that you would have spent anyways. I've already used most of my miles to buy two flights each to Cabo and Cancun this year. I think most sky miles users use their miles for first class upgrades, that is above my pay grade though.
Credit cards are no good if you're going to run a balance. People that own small to medium size businesses are spending upwards of 50-100k+ per month, all miles.