Forgive my ignorance, but what exactly does swiftkey offer over the usual keyboard? A quick google seems to bring up "better predictive text" and "better multi-language support" - is that it?
For me this is half of it, it's also performance. It is MUCH faster at bringing up word suggestions. As in faster than I can even reach my thumb up to type the next word there are already excellent suggestions available, and I often can type entire sentences only touching keys on the keyboard a few times.
Just the performance difference makes the iOS builtin keyboard predictions almost unusable for me (in addition to the fact that the predictions are simply much better on swiftkey).