It's much harder to hear "eight" and mistake it for another number, but "P" and "T" are commonly confused (presumably people would read it as "Papa" and "Tango" to avoid confusion). What about "O" and "0", "I", "l", "1", etc. Now we have ambiguities that may not be interpreted correctly, even with phonetics.
FWIW they left O and I out of the list of possible letters for just that reason.
One could argue either way about fully-numeric versus alphanumeric codes. They're a bit harder to communicate but more than three times shorter, on average.
Edit: math is hard. One and a half times shorter, on average.
It's much harder to hear "eight" and mistake it for another number, but "P" and "T" are commonly confused (presumably people would read it as "Papa" and "Tango" to avoid confusion). What about "O" and "0", "I", "l", "1", etc. Now we have ambiguities that may not be interpreted correctly, even with phonetics.