I refer to Nur Rashidah's letter "Non-Mandarin speakers lose out in job market?" in The New Paper on 26 Mar 2008.
I think it is important to distinguish between preference and necessity, and blind equality and common business sense. I believe that most companies that stipulate prospective interviewees to be proficient in Mandarin are doing it out of necessity and are exercising their business sense. Although English is taught as first language in schools, it is also common that many Singaporeans are more proficient in their mother tongue. Given that there are over 70% Chinese in Singapore, it is fair to assume that a fairly large number of these companies' customers or clients are more comfortable in speaking Mandarin. It would be remiss of them to ignore that telling statistic and not ensure that most of their staff are proficient in their customers' lingua franca.
It is true that this requirement does not "benefit" non-Mandarin-speaking job-seekers, and they probably are losing out. It may also be "unfair" to them, but until Singapore truly manages to make English its first language in reality rather than ideology, I'm afraid this is likely to perpetuate.