There is some validity to the basis of the argument put forth. In an efficient market scenario where there are no events of any type during the day, the price reached during the first 30 minutes can be indicative of the price for the remaining of the day.
The first 30 minutes is usually spent trying to balance all the orders, depending on the volume. Once this is done, a good 'price' for the day can be determined.
However, should there be *any* event during the day: Data, Systemic, Political, Corporate or even Insitutional, the price will then fluctuoate afterwards.
So, in theory, in a vacuum, it could be accurate. Perhaps it would be useful mechanism 100 years (or maybe even 70 years?) ago, when there was little additional information made available during the day. But nowadays with the constant influx of information the price can change dramatically.
It is possible to occasionally see what is being suggested nowadays, but that is more an indicator of a slow news day. If one can forsee days where there will be no events of any sort than one can more accurately determine the price. But then, that is almost the definition of an efficient market.