For starters, that fifth dimension is arguably an interesting dimension in and of itself, in terms of shedding light on someone's personality. Just because Jung and Myers missed it doesn't mean it makes sense to ignore it. Being Limbic (to use the SLOAN term) rather than Calm — i.e., being above-average in Neuroticism — tends to be associated with, among other things, anxiety/worry-proneness; emotional sensitivity/volatility; proneness to annoyance/irritation; self-consciousness; and (sometimes) depression. I'm Limbic (76th percentile on that test), and it makes me less of a cucumber than some of my fellow INTJs.
But another potentially good reason to get a read on somebody's Neuroticism score is that it can sometimes muddy the waters when you're trying to figure out what someone's MBTI preferences are. In particular, in my experience, people who are Limbic sometimes feel like they're "too emotional" to be, e.g, an INTJ. As another example, an introvert is more likely to consider themselves "shy" if they're both introverted and Limbic.