I don't know if it's necessary for everyone, but it is definitely necessary for me. I use witch hazel toner (cheap! like $4 for 16 oz at the drugstore) and it does wonders for my skin.
I have come to the conclusion that for me it may not be necessary as I use a foaming cleanser that you rinse off anyway. Also I have no residue from the cleanser and because I rinse off, this probably closes the pores as well. When I finish my current bottle, I am going to try without!
I don't use one, and for me it's not necessary. I did use toner for years, but noticed no difference in my skin when I stopped using it, so now I don't bother.
Well for me, going without toner I still feel "dirty" and even just looking at my face, it looks that way too. So for me toner is a must, but like others said everyone's skin is different. What works for me may not work for you.
I use a toner and it seems to make my skin look and feel much better. Certainly makes a big improvement to my pores. At times when I've run out and haven't used it for a few days I can definitely see the difference.
I suppose it really just depends on what your skin is like. If you have no problems with your skin without it then I'd see no need to start using it.