Loads of merits here, extra track time for the riders, extra value for money for spectators (or chance to nip to the loo etc if they don't want to watch) cost reductions for entry riders, reduced travel the list goes on.....
Negatives and probably the biggest is the risk of delay/abandonment to the main meeting due to an accident in the match races. Also the standard of rider tracked needs to be looked at, there was a point when riders found their own level and progressed up accordingly but in more recent years there have been some development league riders who are incredibly wobbly and a risk to themselves and others. An "easy" fix would be an assessment of rider standard at a track post meeting. Can the rider circulate within x time of the main meeting, can they slide the bike, do they look in control?
If not then its back to practice days until they can meet the level. Setting the bar lower continually for the benefit of inclusion isn't doing the sport any good.