Yes, reliability should have been dealt with by the judge already. But if two experts appear in front of the jury, and only one uses this phrase, the concern is that the jury takes the statement as proof that that expert's testimony should hold more weight.

The phrase shouldn't be a requirement because it has no legal definition, nor has a method been developed or defined to determine how to determine the level of certainty or what is acceptable or reasonable. It should not be taken as proof that an expert has used the scientific method just because they said so or because they used this phrase. The expert's report and testimony should demonstrate how they followed the scientific method and arrived at their conclusions given the available data.