@dobie - You are right that lymphocytosis is not much of a diagnosis. However, a lymphocytosis differential diagnosis can be extremely valuable in trying to place all the puzzle pieces together.
Abnormal white cell counts can tell a doctor where to look for the problem. It helps to rule out other possible causes so you narrow down the options.
We probably won't ever know everything there is to know about our immune systems. Personally, however, I think we've made great strides and the ability to interpret abnormal blood test results has no doubt saved lives.
So while lymphocytosis on its own may not tell you much, it could be the final puzzle piece that leads to a definite diagnosis. Nothing is irrelevant when it comes to the human body.