Some details in this article -
Using a Quick Charge 2.0 charger will reduce the HTC One M8 charging times
According to the article, fast charging is better for the battery (I always believed it was the opposite, but admittedly don't know a lot about current battery tech).
Re: HTC shipping them with non quick charge 2.0 chargers, call me cynical but ,its probably due to cost!
I've read up a lot on Li-ion and Li-po (Lithium polymer, basically the same thing but different construction technique) batteries from getting into the r/c hobby, of which the M8 reportedly uses a Li-po from everything I've seen. Batteries can only be charged/discharged at a certain maximum rate known as the C rating. At 2000mAh, for example, a 1C rate would be 2000mA (a one to one ratio if you will) 2C would be 4000mA, etc. When it comes to charging, these batteries could typically be charged at a max 1C, regardless of the discharge rating. Just recently, at least in the r/c world, batteries have been coming out rated at 2C and 3C charge rates, greatly reducing charge times. This would have to be true in any battery in a Quick Charge 2.0 setup, meaning the M8 battery better be designed for the higher rates, or it could explode during charging at higher levels. I'm sure it is if they release OEM QC2 chargers for it.
That being said, nothing in all the reading I've seen even suggests that a battery rated for faster charge rates will benefit from doing so, for longevity or otherwise. I've read that in some of the earlier design higher charging batteries, their life *may* be reduced, but that wasn't conclusive and I'm sure technology has advanced a fair bit since then. So I think as long as the battery is rated for higher charging rates, then doing so shouldn't impact the life of the battery either way as long as the battery itself is healthy.
I'm by no means an expert on the subject, it's just battery care and charging are a huge part of the electric r/c hobbies. People will spend just as much or more on batteries and charging setups as they do on the model they are running in, both for longevity of the batteries and safety in using and charging them. Since the battery chemistry is the same as in phones, much of that advice translates to our batteries as well.