The problem with supplementing with formula is this: It will hurt your supply. Breastmilk production is the ultimate in supply and demand. If you start giving formula at night to get more rest, that will lessen even your daytime supply. If you start a very young baby on a bottle, she may well refuse the breast for the simple reason that it's easier to drink from a bottle than from a breast (you have to
work for the milk from a breast; milk from a bottle just drips out).
I was formula-fed as an infant, and I must say that I am a robustly healthy individual--I don't even remember the last time I had the flu, for instance, and I never get a shot. This does not, however, put the lie to the health benefits of breastfeeding.
I'll once again recommend the AAP's statement on
Breastfeeding and the Use of Human Milk, specifically the section
The Need, which outlines the health benefits for mother and child.
Of course, every woman must do what is best for herself and her family, and I will freely admit that I am
not the person to look to for assistance in quitting nursing, as I've been doing it for very close to four years, nonstop (I am, in fact, tandem nursing for the second time).

That doesn't mean it's easy for me, that means it's that important to me.
Any little bit helps, but the longer you can do it, the better.