I like the idea of such a cap and the benefits that it brings in terms of broadcasting, team development and tactics, however our sport is already very physically demanding, with intense tournaments held over weekends or just a few days, and adding more physical expectations in the form of forcing players to play more often will also lead to more fatigue, and more injuries, which further compound one another and result in lower quality of games.
I also agree with Gearoid that it risks decreasing participation as players do not make the squads they want and can not financially afford to be registered with multiple clubs (think insurance, federation fees etc) or devote time to multiple clubs.
It may even have the opposite effect in that top clubs who are monopolising players will just end up entering more teams in national tournaments, as players will still prefer to train with the top players and coaches.