I agree only to a small extent.
Yes, a common market was a reason to merger but it only stands at a small percentage of importance as there were also many other favourable conditions that the merger would offer.
One of another major reason was the political advantages that Singapore would be able to enjoy after merger. While internal self government was previously granted in 1959, there were still some areas like defence and internal security that were still under british control. Since Malaya was a independent nation, Singapore could achieve complete independence after merging with malaya.
Another reason was partly because of fear for Singapore to be fallen into communists' hands.
"Without merger, power in Singapore would be passed to the radicals. This will lead to a Communist base right in the heart of Southeast Asia with serious consequencesto the whole region. Knowing this, it is not unlikely that the British may prevent an independent Singapore from ever emerging."
It was taken from the Straits Times, 19 june 2005 and this showed us that political reasons were taken into consideration and that it was the most important reason. Tunku Abdul Rahman, the Prime Minister of Malaya at that time was also worrying about Communism. Initially, he opposed te idea of merger, but considering that merger would help him deal with communist threats and that the problem of Chinese population more than the malays solved, he decided to merge with Singapore, Sarawak and Sabba.
Of course, the Common market was also something that Singapore hoped to have with Malaya to improve on Singapore's economic growth. The PAP government felt that a merger with Malaya would bring about rapid economic growth. With this merger, Singapore hoped that a Comon market would be set up to support SIngapore's new industries. Within this Common Market, goods could be bought and sold freely without being taxed. This would increase trade, expand industries and create more jobs.
Lastly, setting up a Common Market is not the most important reason but the political reason. It was so clear that the people at that time yearned for complete freedom and to see their nation freed from colonial rule.
