How common are offensive words in tournament Scrabble play?
Short answer: HO is used rather often. LEZ, ABO, GOY are the next most common.

Methodology: I have a database of about a million games played on a popular internet Scrabble server played under the standard Tournament Word List. I scanned this for all uses of offensive words. Roughly 360,000 of the 27,000,000 moves used a word from the expurgated list, or 1.3% of moves.

This table lists the top most-used expurgated words, sorted by the number of times they were played. The rating here is the average rating of the player when the word was used. All the words on this table represent 90% of the uses of offensive words in "average" play among enthusiast Scrabble players.

Word  |   Uses   | AvgRating|    % all 
  HO  |  148182  |     910  |   41.01%
 LEZ  |   26387  |     947  |    7.30%
 ABO  |   25423  |    1043  |    7.04%
 GOY  |   22458  |     936  |    6.22%
 YID  |   16771  |     984  |    4.64%
 PEE  |   14862  |     906  |    4.11%
 JEW  |   12065  |     926  |    3.34%
 WOP  |   11739  |     999  |    3.25%
 WOG  |   10105  |     975  |    2.80%
 POO  |    7257  |     947  |    2.01%
FART  |    5532  |     870  |    1.53%
 POM  |    5075  |     955  |    1.40%
JEWS  |    4211  |     910  |    1.17%
 HOS  |    3482  |     918  |    0.96%
SHIT  |    3200  |     876  |    0.89%
TURD  |    3018  |     943  |    0.84%
DAGO  |    2900  |    1032  |    0.80%


HO is obviously strongly represented by virtue of being a two-letter word; appearing in 41% of the 1.3% of offensive moves means it's played in about 1 in 5 games. You can see some vocabulary effects in the data -- ABO is not natively in many people's vocabulary (this is mostly an Australian thing to my knowledge), whereas FART certainly is.

Other trivia about uses of these expurgated words:


The next word on this list starts with C and you can probably guess what it is
The first seven-letter word is FARTING (169 uses)
240 of the 324 expurgated words were used
The word NITCHIE was played 7 times, but only ever by people rated above 1450 (strong player)
No word longer than 8 letters long was played