http://www.channelnewsasia.com/mobile/singapore/cross-island-line-could/2577450.html
Cross Island Line will be needed, if it works as express service of East-West Line. Conventional trains on East-West Line runs too slowly for passengers moving between Jurong and Changi. So, East-West Line can be alternative rail line.
If you are concerned about budget, setting rapid trains on East-West Line and arranging facilities to make rapid trains overtake local trains would be the cheapest way to get similar effects.