[frankus]

81.JW4 is a lot easier to tell someone over the phone or transcribe from a business card or billboard, and it's pretty unlikely that the US-AK part would be ambiguous in those contexts. And if it is, a simple "which 81.JW4 did you mean?" would probably disambiguate pretty effectively.

[Alupis]

Well, phonetically, pure numbers are better.

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.

[frankus]

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.