Compounding interest of SP is very real. But many people who try to calculate it put the decimal place in the wrong place. Math is hard for some folks?
With just over 1200 SP, I am earning about 0.5 SP interest per week. I did a post a week and a half ago that had some interest estimates:
https://steemit.com/milestone/@preparedwombat/1-000-steem-power
Anyone who thinks that SP does not earn interest needs to look at the wallet page of 
RE: 💰💲💸 No, just holding STEEM POWER won't make you a MILLIONAIRE so easily!💸💲💰