Not to mention that once the vouchers expire all the chapters that you have read are locked back again.
This doesn't include chapters that you paid in coins like the discount vouchers. they are unlocked forever. So which one is better?
1) Free X-days Site-Wide Vouchers
2) Discount Vouchers
I also notice they added Fast pass vouchers
Also the other one confirmed by CKtalon, Is redeemable only once per users.