The critical distinction between explanatory and pragmatic trials is not regarding whether a trial produces useful information. Rather, it is what use that information is for specifically: pragmatic trials are those aiming squarely at therapeutic usefulness in the clinic.
The Pragmatic-Explanatory continuum was first proposed by Schwartz and Lellouch in a 1967 paper titled "Explanatory and Pragmatic Attitudes in Therapeutic Trials" in the Journal of Clinical Epidemiology, and which was cited by the Precis-2 developers. In this article the authors present two randomized control trial scenarios testing in an anti-cancer context a drug preparatory to radiotherapy vs. radiotherapy alone. The drug is administered 30-days prior to radiotherapy in order to sensitize the patient to the effects of radiation.
The drug for 30 days followed by radiotherapy is tested against a 30-day wait plus radiation
The drug for 30 days followed by radiotherapy is tested against radiation alone beginning immediately
The first scenario, which they describe as explanatory provides "information on the effects of the key component," while the second scenario, described as pragmatic "compares two complex treatments as a whole under practical conditions".
Schwartz and Lellouch give another example distinguishing explanatory and pragmatic trials: a randomized trial where two analgesics of very similar molecular structure are compared on an "equimolecular" basis is explanatory because it is interested in the relative effect of these drugs based on the same dose; by contrast, two analgesics with radically different structures having different "optimal levels of administration" are best studied using a practical design, intended to compare the optimal effectiveness of each treatment.
The authors summarize:
The “comparison between two treatments” is a problem which is inadequately specified even in its over-all characteristics. It may imply one of at least two types of problem which are basically different.
The first type corresponds to an explanatory approach, aimed at understanding. It seeks to discover whether a difference exists between two treatments which are specified by strict and usually simple definitions. Their effects are assessed by biologically meaningful criteria, and they are applied to a class of patients which is rather arbitrarily defined, but which is as likely as possible to reveal any difference that may exist. Statistical procedures used in determining the number of subjects and in assessing the results are aimed at reducing the probabilities of errors of the first and second kind.
The second type corresponds to a pragmatic approach, aimed at decision. It seeks to answer the question-which of the two treatments should we prefer? The definition of the treatments is flexible and usually complex; it takes account of auxiliary treatments and of the possibility of withdrawals. The criteria by which the effects are assessed take into account the interests of the patients and the costs in the widest sense. The class of patients is predetermined as that to which the results of the trial are to be extrapolated. The statistical procedures are aimed at reducing the probability of errors of the third kind (that of preferring the inferior treatment); the probability of errors of the first kind is 1.0.
Schwartz, D. and Lelluch, J. (1967). Explanatory and pragmatic attitudes in therapeutic trials. Journal of Clinical Epidemiology, 20:637–648.