I am not KJV-only, but I like & often prefer using the KJV for these reasons:
1. The translation is one of the only mainstream translations which is translated from a representative of the majority of Greek texts. Most mainstream English translations from the late 1800s onwards are based on a minority of Greek texts.
2. The older wording preserves the differences between singular and plural persons;' e.g. "thou" (singular) or "ye" (plural) would usually be translated "you" in other more modern translations. Thus, there is a loss of precision in more modern wording.
3. The rhythm and cadence of the KJV text lends itself to memorization.
4. There are many more books, commentaries, and helps based on the KJV than any other translation.
5. Reading the KJV causes me to slow down and think harder about the meaning of the verse, whereas more modern translations are "easier" to read and can cause me to breeze through the text. I see the former as a positive, because I find myself meditating more deeply on Scripture.
6. Reading the KJV "elevates the mind". Science has shown that reading or attempting to comprehend complex texts/words/music causes the cerebral cortex to activate much more greatly than reading easier texts/words/music. This leads to greater development and connections in the cerebral cortex. Why is this important? The cortex is involved in higher brain functions, e.g. memory, rational thought, logic, patience, self-control, etc. A weaker cortex causes the "lower brain" to dominate more in an individual, e.g. impulsiveness, fear, depression, etc.