There's also a reason you say longer terms over radio. Things like the phonetic alphabet because you want to make sure that you're heard.
Not quite. It's because many letters or words sound similar and could easily be confused, especially where heavy accents or back ground noise are present. For example, "no" could easily be confused with "go". B, d and e sound similar, as do m and n. S and f can be hard to distinguish on the radio. E and i can get confusing with conversations between French and English speakers. Even the numbers 2 and 3 are often indistinguishable, amazingly, with foreign accents. The phonetic alphabet and numbers and the use of standard international phraseology are critical in multi-national RT, but I have to say that US pilots are terrible at it. They drop numbers from read-backs, use non-standard terminology and often don't bother to read back the details of critical clearances, responding with just a callsign or "roger" (illegal response). They're not alone, by any means, but it is a predominant disposition. But then again, BSG's continual and undisciplined RT chatter and distorted sound may be more representative of US RT - European and certainly British RT use tends to be stricter and clearer, especially UK military, which is generally free from distortion or garbling and is fairly strict on what is actually said.
But "negatory" has never been used outside the US and, frankly, sounds ridiculous.
By the way, my reference to George W was tongue in cheek - I hadn't heard him use "negatory" but it does fit the pattern of horrible non-words and mispronunciations that he would use in public speeches.