Disinformation, disinformation, disinformation. That's all you oppos know. You should go and write for the Sinapore Democrats.
The PMO is also a ministry with subjects. With more subjects assumed by the PMO, more ministers are needed. It is the same in Malaysia that is why the de-facto law minister is minister in the Prime Minister's Department.
SM Goh has his own subjects. SM Jayakumar has also got his own subjects. The two DPMs also have their own ministries to run.
Lim Swee Say fills a role which has been filled since the days of Lim Chee Onn - labour leader with a cabinet voice. He could have ben like Lim Chee Onn or Ong Teng Cheong - Minister without Portfolio. But PM Lee decided it would be better to park him in PMO.
Of course if you prefer, PM Lee can split PMO into 6 more ministries. Then PMO need not have so many ministers.
But all this does not matter to oppos. All they want are criticism opportunity.