Now that I explained how much will be lost without free rerolls, let's discuss the reason to introduce it at all.
I asked Cirn0 "Why", he said he would have thought about it and never answered back.
One of the obvious reasons you can see is such called "economics".
Let's be honest, the amount of the bugs/features/events which made current economics completely broken is huge, some of them haven't been fixed yet, although known for months. Not to mention that developers said something like "We threw paper planes with item names down the stairs and the further they got, the higher the price was set". Not to blame anyone or anything, but incoming "tier progression system" just can't be balanced economically soon enough, while old bugs still gives you free money, old economy never was close to be balanced or at least stable and the system in whole is highly abusable.
There are thousands of different controls to balance the economy besides purchasable rerolls, the reroll by itself won't even be used in the combat, it's not a thing you can directly use to play. It by its nature is much closer to the information, which shouldn't be sold to players or hidden from them.
Basically, there are 2 ways players could or will have to go, depending on rerolls are free or not. Spending time grinding, preparing new characters, or playing 1 character and rerolling it adjusting to the situation. And multi-charating is just bad for so many reasons.
We played without rerolls for quite a long time, we played for a long with partials rerolls. Many of my friends spent most of the time grinding in levelling, creating new characters to figure out build science and gear balance. I don't think that anyone would like to do that again, after we realised that we can just play and just have fun. When we had no free rerolls, the question "when new rerolls available" was one of the main in IRC channel as the game was never balanced, but it stopped to matter as soon as full free rerolls were introduced.
So, why? And more importantly, does it really worth it all the benefits? Or is it just another "too lazy" excuse?..
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Later we will talk of the cost of introducing purchasable rerolls.