the 55-200 VR is a great portrait lens. It's versatile, offers better foreground-background separation than the large-aperture primes and is less brutally sharp too. Great color rendition for skin tones and very inexpensive to boot.
The range itself, allows you to be outside your models' "body bubbles". Even if you are comfortable being in their space, the opposite may not be true and that's where the 100+ mm range comes in very handy.
It also allows for more candid shooting since you may go unnoticed from a further distance.