I, for one, fail to see the advantage of PMR. It'd be much nicer if there's no streaming. What's arts and sciences students? It's so redundant for students to be asked to follow certain streaming, to choose a 'stream' with fixed 'choice' of subjects at a quite young age (which'd possibly affect their future undertakings). Wouldn't it be nice if students at secondary level are given the chance to pick the subjects that they're going to study, mix and match em to suit their passion? Not to mention the societal/parental stigma & expectation of each of the arts and sciences stream hence the burden it poses on the students.
For UPSR, its justified I guess to have a centralized exam after the end of primary years, as it is relevant to have SPM as the centralized exam marking the end of secondary years.
My verdict: Reshuffle the whole education system, esp the lower secondary level, and abolish PMR alongside the streaming system.

But looking at the state of Malaysia right now, that'd be wishful thinking at its best...