I agree with Dragon that it's just tacky and with escoville that it really shouldn't be pronounced as 'ex-mas,' except perhaps in fun.
Dragon's point about levels of formality may help explain why, as bluesky's quote shows, many people feel the abbreviation is sloppy or disrespectful. If you want to save time by writing 'Xmas decs' instead of 'Christmas decorations' on your own shopping list (or in your own txt msg), it makes perfect sense. (And there, the hyphen is really unnecessary, since the whole point is to save time and space.) But if you used the same style in a typed or printed text to be read by other people, it would just look bad.
Similarly, if your store has one of those big signs out front with three or four short lines for magnetic letters, and you normally put up abbreviations like '2 FOR 1 SALE ON LG PUMPKINS THRU NOV 22' (= large, through, November), then of course it's fine to write 'JUST IN: XMAS TREES & WREATHS' as well. But most contexts don't require such drastic shortcuts.
escoville is quite right to point out that X (= the Greek letter chi
) has long been a shorthand symbol to shorten words like Christ, Christian, Christianity, etc. Of course the Roman and medieval scribes who used it also used all kinds of other fascinating little abbreviations
and shortcuts that we no longer use, because we don't have to write entire books by hand.
However, things like 'Xtian' and 'Xty' are still convenient for taking handwritten notes on topics in religion, art history, etc., just as 'C13' is a convenient shorthand for '13th century.' (-: