Military and civil R&D expenditure

Military R&D is usually tied to specific aims unlike many civil quests for knowledge — even if their interests overlap. Welfare and military research can compete politically, but military R&D is rarely a successful route to economic and social benefits. About one-quarter of all world R&D expenditure is of a military nature and is concentrated heavily in a few countries. This spending is often justified on the basis that military R&D can also have civil applications but is this a valid argument?