Question 367857
We could use only pennies.  It's a matter of convenience, I suspect.
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A better question is, Why do we continue to bother with pennies?  They're not worth picking up off the street.
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Plus, they cost more than $0.01 to produce, $0.0146 last I heard.
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Another factor is the common use of base 10 for counting.
Almost all oscilloscopes offer time and voltage range selections of x1, x2, x5 rather than x1, x2, x4, x8, x16, etc.  This keeps the units in base 10, volts, millivolts, microvolts, and time in seconds, milliseconds, microseconds, nanoseconds, etc.
Binary is easier and simpler to deal with in computer applications, but humans prefer base 10 for counting.