A letter from your MP will provide you with a little help in getting your PR application approved. This is because such letters are merely general in nature and does not indicate your key strengths as a candidate for PR. Further, in order to get a letter from your MP, you must be active in taking part and organizing activities in your neighbourhood for some time as part of the social and community integration.
However, candidates who have been working or involved in key national or government projects, and have a personal endorsement from officials in key positions, including MPs, definitely stand a better chance, however the documentation requirements is different from that of an MP letter.
If you are serious about attaining PR, you should also understand that the Singapore Government exercises extreme meritocracy in according approvals and will take into consideration other key components that highlights you best traits as a candidate.