In adaptive immune responses, lymphocytes are responsible for immune recognition, and this is achieved by clonal selection. Each lymphocyte is genetically programmed to be capable of recognizing just one particular antigen. However, the immune system as a whole can specifically recognize many thousands of antigens, so the lymphocytes that recognize any particular antigen are only a tiny proportion of the total.
I'm not sure what the question is. Because the question is flawed... This implies that there is someone with a normal T cell population that recognizes no pathogenic antigen. There isn't, is there? Thus, there is no advantage.
The advantage is generating a bunch of T cells that can recognize a wide variety of antigens to prevent antigen escape. That whole process of activation of naive T cell, generation of memory, etc. is a way to get your immune system to recognize a bunch of different antigens at any one time, BUT still capable of mounting a quicker response after initial exposure.
The problem is, nobody tells the immune system just which cells recognize a pathogen and which ones don't. It has to discover it experimentally, by randomizing a T-cell or B-cell receptor and testing it out. To test many possible variations out, it first has to synthesize them. There is no way for it to know "in theory" which ones will work.
The other arm of the immune system, the innate arm, works differently. It has during billions of years of evolution acquired the ability to bind known pathogen sequences, that have been common throughout history. For example, all neutrophils express the same TLR4 receptor, which binds bacterial LPS. There isn't that perceived "waste". You won't find a neutrophil that can't bind it.
But the price of that is that the neutrophils can't "learn" to target a previously unknown sequence. All they will ever do is take out bacterial LPS. If a pathogen was to evolve that manages to hide its LPS, or doesn't have it in the first place, such as a virus, then the neutrophils can't do anything about it.
So you want both methods. Some level of "waste" is acceptable, if it buys you the ability to learn completely new tricks.
The evolutionary advantage of such diversity is to prepare the immune system to protect mammals from virtually any infection that we may face during our lifetime. For those infections that we cannot eradicate immediately, we have evolved mechanisms to dampen the immune response so that we can live with such chronic infections. Of course, the downside of such a vast repertoire is that some people develop allergies and some autoimmune diseases. We all have receptors specific for allergens and self-antigens but only a fraction of people have all the immune genes disposing either to allergy or autoimmunity. I would argue that those people susceptible to allergy or autoimmune diseases are more likely to survive infection with 'super' microbes hence protecting the survival of the species.
https://www.sciencedirect.com/science/article/pii/B9780123852458000017
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