Like someone mentioned earlier, it takes almost 2 years if not longer to train a pilot, and even then he still needs another year or two to become truly productive (i.e. making money for the company). All in all, this could be somewhere between 3 to 4 years.
Notice that the pilots being asked to take leave are relatively senior as compared to the trainees. The duration is somewhere between 6 months to 1 year.
Combine this two facts and you will notice that SIA expects that this year, the aviation industry will be slow in general. They are also expecting an improvement in the coming years. This is supported by marco economic trends such as US posting better and better economic indicators and EU slowly pulling themselves out of the debt mess. Last but not least, the prospects of oil in Brazil and Africa would provide an increase in air traffic demand.
Hence, it is imperative to start recruitment and training now to meet future demand needs, while cutting costs in the interim. Personally I think they are trying to save up as much cash as possible so that when demand rises, they can afford to purchase the necessary equipment without relying too much on external finance, which can get very expensive. Moreover, having cash on hand would mean the company will be in a better position to mitigate raising costs, especially given how oil prices rise with better economic prospects.
TL : DR version: SIA is doing so to meet both short term and long term needs.