Unfortunately, Guest Riders remain a necessary evil for speedway, and as much as I dislike the system I have to admit that I can't think of a workable alternative which would do away with the need for them.
As many people point of, comparisons with football have no relevance. A less talented footballer replacing an injured colleague can be adequately covered by his team-mates with a slight change of playing style. If he isn't up to scratch, then his team-mates try to keep the ball away from him or position him were he can't do too much damage! Football is a resuts orientated sport - for most supporters the final score is all that matters. Whether the match they have watched was entertaining or not matters nothing, but speedway is nothing if it is not competitive and the teams need to be reasonably well balanced to achieve this.
My gripe with guest riders is really the way that promoters seem to regard it as the first option and not the last. Both guest rider and rider-replacement rules seem to favour the already strong teams and handicap the weaker ones, so a possible solution might be for a guest rider's average to be only enough to take the overall average of the team for whom he is guesting, up to the points limit instead of as at present, allowing a guest with an average up to that of the rider he is replacing. This would create a situation in which rider-replacement would be a better option in many cases, and although rider-replacement isn't a favourite rule for many supporters (including myself) I still prefer it to guests.