Full disclosure: I have two kids who are four and two years old. My four year old stopped nursing just before she turned two, and my two year old is still breastfeeding, though we are slowly working towards weaning. I wear a D Cup, and have to confess that I have never warn, or even actually seen a nursing bra. I have seen pictures of them, however, and know how they work. I'm have nothing against nursing bras, but I am the living proof that nursing mothers most definitely don't need them. The basic feature of nursing bras is that you can open that part which normally covers the nipples and most of the breast. You can open and close these "pockets" with the help of buttons, snaps, or clips. Because underwires are said to be bad for breastfeeding mothers by some, many nursing bras do not have underwiring.
Nursing bras might just be better for those women who prefer nursing their babies by lifting their t-shirts up towards their necks, rather than pulling them down to uncover their breast. I prefer the latter method, and can't imagine how opening a nursing bra "flap" could be any more convenient or easy than just taking your breast out of a regular bra. That is what I have always done, and it could literally not get any easier. The hassle of having to open and close the pocket in a nursing bra does not appeal to me at all! Having said that, many mothers love nursing bras and say they could not do without them. If you are curious enough to want to try one, but not entirely sure you will use it, buying just one would be a great start! For other breastfeeding topics, look at enlarged breasts your body is preparing for breastfeeding and early pregnancy signs while breastfeeding.