See related posting regarding the current role of journals and where to submit (i.e., to which conference and/or journal).
My basic thesis is that conferences are intended for the present, whereas journals are intended for the future. That is, conferences are intended for publication of ideas and/or results that may be useful to the relevant research community at the current time (i.e., today), whereas journals are intended for archiving ideas and/or results that may be useful to the research in the future (e.g., in five years). In other words, conferences are intended to serve the current exchange of new ideas (and results), whereas journals are intended to serve future generations that may need to know old ideas (and results).
While there is a considerable intersection between these two types of ideas (and results), these sets are not identical, and this assertion should not be viewed as diminishing the importance of any of the two sets. On the one hand, there may be ideas and/or results that may have an immediate impact on research of others, while not meriting archival. Typically, these are incremental ideas (or results), which lack the importance of pioneering a direction, yet push this direction forward and are useful for subsequent research (which are likely to deem them obsolete). Such ideas and/or results are naturally and justifiably presented in conferences, but need not be archived in journals.
On the other hand, results that "close" a direction are definitely publishable in journals, but they need not necessarily be presented in conferences (e.g., if the direction was "almost closed" before and/or if there is little current interest in it). For example, interesting results that indicate a dead-end in a natural direction that nobody is currently exploring may fit in a journal but not in a conference.
An immediate corollary of this thesis is that it is wrong to draw inferences between the evaluation of a paper wrt possible conference presentation and its evaluation wrt possible journal publication.
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