Well, just as I moaned last week, I'll do so here as well:
Free limited reading is great... if... IF... your readers have all the time in the world to be able to drop whatever they're doing to read their favourite novel at the exact time it's free-to-read.
HOWEVER, you may be surprised to hear this but some of us can't do this - some of us may have other things to do!
π² Shock! Horror!
As such, some of us would rather purchase chapters in advance to read at a time more suitable to them. These people like discounts, because these people know they'll actually be able to get guaranteed benefits from discounts. That goes for everyone, whilst free reading is only beneficial to a subset of your users!