While there are no doubt some issues with a tiny proportion of landlords withholding tenant deposits getting the PRTB to hold deposits is impractical for tenants surely.
For tenants moving how will they get a deposit together for their new landlord? Landlords normally give the deposit back as a rule when getting the keys back at the end of tenancies, tenants simply hand the money over to the new landlord.
Will the PRTB hand it over immediately to tenants? I doubt it. Presumably there will be more paperwork involved for all. Hence a delay in tenants getting their money back. The delay may well put tenants under pressure in not having enough money for a deposit and first months rent on a new place.
Probably well intentioned, but perhaps a sledges hammer solution? Not very practical in my view. The solution will need another solution methinks.