Spanish Phonetics

No, I am not talking (or writing) about my Gringo accent that will not leave me. In Spanish speaking countries it is not uncommon to see signs for "ceviche" at one restaurant but at another they serve "cebiche". The reason for this is that the "v" and "b" sounds in Spanish are so similar that many people confuse them. In fact, in many conversations I have been part of, a native speaker asked, "Is that with a large or small b/v?"

Also, the "s" and the "z" sound freakishly identical. My primary question: why have two letters that have the same sound ALL the time. Sure, in English the "s" and "c" sound the same depending on what follows the "c" (as in 'cell'). But not always is the sound like the "s".

Am I missing something here (like I felt when watching these two guys at the table next to me at Los Yuyos restaurant...until I learned this is the custom followed for a soon-to-be-husband)