The clue writing competition has been a regular feature of the Ximenes/Azed puzzles in The Observer for the last 74 years and has produced many of the most memorable clues ever written. Inevitably it tends to favour the significant number of crossword setters who take part, some of whom have achieved multiple wins, but the winner of April's comp had been taking part in the clue writing competitions since 1959 without having previously lifted the trophy.
75% of the year's Azed puzzles offer everyone who completes and submits the puzzle an equal chance of winning, so I don't see that adding a further test of skill to the remaining 25% is unreasonable - and please note that Azed himself reads every clue that has been submitted.
Incidentally, many of those who enter the clue writing competitions also complete the non-competition puzzles but don't submit their entries, thus improving the chances of those who submit only the non-competition puzzles.