I believe that the minimum height for posts (if I researched correctly) is 3.4 m, with crossbar at 3 m. When playing at small club or grassroots levels you'll be hard pressed to find a team that can manage 18 m goalposts. If you standardize the height, then you're going to see poorer third world clubs unable to meet the goalpost height requirement. Hell, some are lucky they even have a rugby ball. Kill the game on that level by making rules that are unrealistic for the clubs at the bottom to meet and it's like cutting out the roots of a tree.
Your biggest concern is that it's hard to discern if a ball going over 18m is within the posts. Is this also a concern of assistant referees? They're the ones who would be most affected. If we're hearing no complaints from them, then it's highly unlikely that high balls are a problem. Since most kicks (conversion and penalty) occur with the eagle-eyed assistant refs standing right under the posts, I don't think they have much of a problem judging if the ball was within the posts. The only place this may cause a problem is with drop goals, since the assistants may not be right under the posts when the kick is made. But relatively speaking, drop goals are rare and extremely high balls even rarer, so it wouldn't be logical to make such an arbitrary change on the odd chance that a ball may go into orbit somewhere around 18 m.
A better solution? For the big games where it would really matter - the Cup, the major competitions, the international matches - just use well-positioned cameras. If they can take those awesome pictures of the moon during half-time, then I think a good camera can also find other objects launched into orbit, especially those much closer to home.
das
