I have heard that the body can adapt to moisturizing creams, thus relying on them to keep the skin hydrated, so maybe it's the same for antiperspirants...
That might be true, I do know that products like Shampoo can build up (hence cleansing shampoos) and certain facial cleansers lose their punch unless you switch to other products once in awhile to keep your skin on it's toes. Same way with exercise.. same thing all the time will eventually lose effectiveness.
In the past couple of years, I've had to change my deodorant brand something like 15 times, because either they burn my underarms, or just don't work well for me. Lately, I get excellent results with Right Guard Sport Stick.
I used to use Old Spice but something in the chemical makeup made my underarms darken. It wasn't pretty. Now I use only KMF's Liquid Rock deodorant. It's basically a scented alum block. It works well and my skin is regaining its color.