Necessary? No.
Sometimes nice to have? Absolutely. If you are not sure about how you will feel re: nursing in public, it would be great to have a couple of tops, I think.
I started NIP the day after I came home from the hospital, and it helped me (psychologically speaking) to know that I wasn't flashing the entire restaurant/doctor's office/library.
My daughter hated to have anything (blanket, sling tail, etc.) over her head, so I loved having nursing clothes when I was at synagogue or eating at friends' houses. I never felt like I had to leave the room or "cover up" (I was already covered).
That being said, I only spent full price on one top (from Boob nursingwear); everything I bought from Motherwear or Expressiva was from the clearance section or end of season sales (when dd arrived in June I had 3 sweaters and a turtleneck, lol). And one very kind friend who ebf her kids gave me a gift certificate to Motherwear as a baby gift. I certainly don't have a full wardrobe (dresses, fancy tops, etc), but they definitely helped me feel comfortable in the first, say, 6-8 months of nursing. After that I didn't care quite as much.
