As far as I know, Apple is still very restrictive with their conditions for what a keyboard can and cannot do, and SwiftKey only asks for that extra bit of access so that it can link with your Facebook, Twitter, Google+ feeds etc. to offer better predictive text.
The company has been around for a while and is quite reputable, and I'm fairly certain that iOS even reverts to the stock keyboard when entering sensitive information like passwords or credit card numbers.
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That being said, I'm a bit surprised that SwiftKey is leading by so much in the poll. Am I the only one having a bad experience with swiping words on it? It almost never seems to get the word correct for me...