We allow dummy to take the status of spectator in order that there is more of interest for dummy to see while having no input to the game(other than possibly to reply to any questions about a call made in the auction). Each individual has an option to restrict their own sight to that of theirs and partner's hands or even just their own hand. Few do this.
If a partnership is going to cheat (ie transgress Law 72B by exchanging information privately), they will cheat whether we allow dummy to see defenders' hands or not.
Our Serious Ethics Consultant, has often made the point, that if a partnership is exchanging information privately, then it is easier to detect and analyse if they are doing so when dummy and declarer than if they can do so only in the auction or defence.
Bridge Club Live relies partly on players reporting (discreetly via our Incident Reporting system) anything they think suspicious, but we also are pro-active ourselves on the Management Team. Cheating is actually very rare in BCL and much of the reason for that is that when a partnership does start to cheat, we catch them quite quickly and terminate their memberships.