I can think of some reasons to use auto-calc.
1. Performance: If you're using a computer with weak graphics capability, battles are all but fun or even beautiful. Take my AAR, for example. I auto-calculated all of those battles except for about three up to the point where it is updated now, but somewhere in there are some battle screenshots which may show you what I mean. Add to that an unnerving lag and you'll be stuck with fighting only those battles for yourself which are absolutely needed.
2. Time: Even not-so important battles can easily take twenty, thirty minutes. If you want to get somewhere with your campaign, but you can only spend a few hours a week on playing, there's no way around using auto-calc one time or another.
3. Roleplaying: You might want to fight only the battles a certain FM - e.g. the faction leader - takes part in, representing that he has no direct influence on any of the other battles. And in combination with 2.: Is it really essential to fight the thirteenth battle of that captain at the end of the empire against some - at best - annoying rebels while the real action is going on someplace else?
I don't see anything wrong about using auto-calc. You save some time, but you'll pay some of your men in return, because auto-calc will almost always give worse results than fighting for yourself (with exception of siege assaults in some relations of strength and perhaps battles against horsearcher heavy armies).
Oh, and if you are using very hard campaign difficulty you can expect to be owned in auto-calc battles, so you'd better only use it on medium or perhaps hard. Auto-calc difficulty depends on campaign-, not battle-difficulty.
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