Basically, a landlord may only retain a part or all of the deposit to cover any loss in the case of rent arrears, damage in excess on normal wear and tear or outstanding utility bills for which the tenant was liable.
The clause is that we agree to forfeit all of our deposit in the event that we breach one of the other clauses.
Without knowing the exact wording of the clause it is difficult to say anything more. What you have written is, I assume, your brief description of the clause.
Taking an example: if the lease agreement has a clause saying no pets allowed and you had a small dog.
The PRTB will only consider any damage that might have resulted in having the dog in the property. This may include actual damage to furniture, floors etc as well as having to have the property professionally cleaned to eradicate any traces of dog hair/smell that might affect a future tenant who could be allergic to dog hairs.
IMHO, in a PRTB dispute, just by having a pet in the house, although a breach of the conditions of the lease, would not, in itself, render the forfeiture of the deposit, under the type of clause that you have described, as the landlord has to be able to prove a "loss".
Again, IMHO, in the case of a clause prohibiting smoking, the landlord may be able to claim from the deposit for repainting and professional cleaning in order to remove any trace of the smoking. Smoke can cling to furniture coverings, carpets, curtains etc, as well as discolouring painted walls and ceilings. The landlord may only retain any portion of the deposit to cover the expenses of remedying the above but cannot blankly state that because the clause prohibiting smoking was breached he will retain the full deposit.