@spendius,
Maybe someday if you lose an ability to speak or write it will make sense to you. If I had tried to learn writing again without certain letters my abilities would have been seriously hampered.
The fact is that the letters, X, Y and Z exist in many words and by removing all words with those letters from play you effectively kill this entire thread. We aren't speaking just of words such as Zigzag, Xerox, xylophone, zero, year, yeast, yogurt, and those, there are also eXample, eXercise, penitentiarY, waYward, siXteen, siZZle, amaZed etc. etc.
Just because your dictionary has ony 10 pages with those letters as starters doesn't mean those are the only words existing with the letters.