Luta Poor you... you haven't figured it out yet. This promotion is for those who don't pay SS for premium chapters. However, once you start, you can't stop. You have two options after you "binge" on discounted chapters.
1) You wait for the free chapters to catch up to where you stopped. However, if you stockpiled a lot of premium chapters... it means you get to stop reading the novel for a while, possibly months. Example: Novel A has 300 chapters free, with another 100 premium chapters. You binge the 100 premium chapters on discount. Now you get to wait 3 months for the free chapters to catch up (assuming 1 chapter a day) to where the premium chapters left off.
2) You can't wait for the free chapters to catch up so... congrats, you now have joined the rest of us mindless slaves in paying SS for every chapter that comes out. You took the bait and now it's all over for you.
The only event that everyone benefits is a free mass release. This discount promotion is only to bait those who are on the fence to spend spirit stones. If you really think about it, the only winner is Qidian.