It is an arbitrary number indeed.
I'd say two thousand and ten, like this year I always call two thousand and nine, not twenty-o-nine or twenty-nine.
People who say twenty-ten and twenty-twelve, do they also say twenty-o-nine, or twenty-nine? I never heard that (or twenty-twelve). But it is shorter.
It is also a matter of language. Maybe that's why I never heard twenty-o-nine or twenty-nine? Or does no one call 2009 that?
Because in English if you would say twenty-nine then you'll get 29 (or 20-9), so probably no one will call 2009 twenty-nine or do you?
In Dutch it works differently. In Dutch twenty-nine would be 20-9, 29 we call nine-and-twenty ("negenentwintig"; just like in German you would say "neunundzwanzig")
And in Dutch saying twenty-o-nine isn't shorter than saying two thousand and nine; it's the same number of syllables. Though twenty-nine or twenty-ten are shorter.
How does it work for the 1900s, do you say nineteen-six or do you say nineteen-o-six?
Nineteen-six is shorter, but nineteen-o-six is actually clearer or more accurate. But if you know someone is talking about a year, adding the o isn't necessary.
2010 is also MMX, 2009 is MMIX, 2011 is MMXI and 2012 is MMXII.
I like Roman numerals.
I don't understand klick's "7DA", "7D9" and "7DB" 
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1975, ASD: Asperger's Syndrome (diagnosed: October 22, 2009)
Interests: science, experimental psychology, psychophysics, music (listening and playing (guitar)) and visual arts
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