Pasteurizing juice will most certainly not 'spoil' it, but it may alter the flavor. Generally speaking, heating something sensitive will make it taste "cooked" instead of "Fresh". IE, baked apple, as opposed to fresh apple. This might be alright for something like apple (depending on preference), but it would probably be terrible for, say, cucumber. (I wonder how cucumber IPA would be...?)
Pasteurizing at lower temperatures may preserve more of the flavor.
It depends on the compound, and the environment (such as PH, sugar levels, minerals, etc), but higher temperatures tend to have a more dramatic effect than long pasteurization times. However, some sensitive flavor compounds may be rapidly lost at any effective pasteurization temperature, as is the case with the highly sensitive oils of aromatic hop varieties.
You can of course skip pasteurization altogether, but going without pasteurizing allows wild fermentation, which could impact the flavor greatly. It's not often done with beer outside of specific styles, but for ciders, wild fermentation is fairly common. Despite the seeming odds, aside from ABV limit, wild yeasts seem to be more likely to exhibit desirable or inconsequential traits than they are bad ones, and can even posses qualities unavailable commercially. However, it is inherently random (it is up to whatever strain dominates the area, in this case the farm's juicing room/equipment.), and you could also end up with bacterial agents instead of yeast, resulting in vinegar, or worse. (As far as I know, the result is generally 'safe' but may be unappealing.)
One alternative to pasteurization would be to use metabisulfite chemical treatments. IE, Campden tablets. Campden is a sanitation agent that dissipates over time. It can be used to, in effect, pasteurize without needing heat. (It is not as effective as pasteurization, but it can give your yeast a head start compared to other populations that may have been present.) Note that you need to wait around 24 hours after treatment before adding yeast, or else the yeast will be impaired. I feel I also should mention that there are some claims of health concerns, as well as people who may be allergic to the sulphites that Campden will leave, so be aware of a potential risk there. It is commonly used in commercial wines, nonetheless.
One final note on ciders... unlike beer, the sugars in cider are very easily fermented away comepletely, leaving a very tart, bone dry product that can taste... wrong. Aging for a few months seems to help, but some amount of back-sweetening may be needed to strike a good balance. Of course, this is up to preference.
