I think they are worth it, but I live in a hot climate where wearing two shirts will broil you in the summer. I don't like pull down nursing shirts or just wearing lower cut shirts because they expose a lot (H/I cups here!).
I also don't like the two shirt arrangement because you can't nurse in a carrier with the bottom strap holding you "over" shirt down.
At home, I just wear regular shirts and pull them all the way up. I have done this outside the home too, the baby just covers most of it, and I nurse in the car a lot. It's a good place with few distractions!
I wear nursing shirts to work and church and usually when out with LO. I pump at work and it's nice to NOT have to take your shirt off or pull it all up above your boobs.
So for me, worth it. I also struggled with finding nursing shirts that weren't "maternity + nursing", so I had to order $$$ ones online, but totally worth it working full time and stuff. Now I'm pregnant again, and I'm on the lookout for some of those "Maternity + nursing" shirts again....