The premise for the proposal: "Dr Richard Vautrey, deputy chairman of the BMA’s GP committee, said the changes were needed to reduce the pressures on doctors." suggests that employees are seeing their GP only for the purposes of obtaining a sick note; not because they feel so unwell that they require medical assessment by a GP.

As a premise, it appears a flawed one.  Even if it were flawless, people who are so unwell to have a period of 2 weeks' sickness absence are more likely to see their GP at the start of that period and obtain a fit note from that point, rather than going back after a period of self-certification solely for the purpose of enabling the employee to satisfy the statutory requirements of obtaining SSP.

Dr Vautrey says employees should be trusted; this is probably true of most.  For those that abuse the system, the  procedures that many employers have in place, such as return to work interviews, will deter some abuse.  

It might be that should this come into effect employers will want to require more frequent contact during a period of self-certification or arrange home visits to support the employee during this period (as they would not be receiving any support or guidance from their GP).  Some might even think about having company sick pay  apply only after the period of self-certification with SSP being paid in the interim.

It's only a proposal at the moment, but employers would be able to take steps to support employees and avoid abuse should it be implemented.