It is perhaps understandable that some people would hold such a position because it seems obvious that millions of people work either directly or indirectly for companies that derive a significant portion of their income from military contracts. Were those contracts to end, one might imagine many of those people becoming unemployed and many businesses simply closing up forever.
In essence, this is a very cold, utilitarian argument because it justifies the existence of war as in institution in order to maintain some desirable end (employment, a strong economy) despite the possibility that an actual war may break out and real people may die. There is no effort to defend war itself in any fashion. War is, rather, just an unfortunate consequence of other objectives - sometimes in order to get things we want, we have to accept things we don't want.
That this argument is problematic is made evident by the fact that hardly anyone actually tries to make it openly. It is a defect of some utilitarian positions that they can be used to coldly defend apparently immoral actions in the interest of achieving some presumably greater good.
The argument that war is necessary to keep the economy going is often put forth as a criticism by Marxists who have long denounced capitalism for encouraging conflict and war among nations. Marxists have also argued that as the ability of capitalists to exploit domestic markets declines, they will seek other markets elsewhere - and capture them by force, if necessary. Although...