Yes I agree infrastructure provides benefits to future generations but there's no point in deficit spending unless it is necessary, deficits of the 2000's weren't really necessary. Some of them perhaps were but most were not.
The tax could potentially hurt the economy some, but thats why politicians MUST consider the consequences of their actions. The theory is that military spending would outweigh the drop in aggregate demand and should therefore be a net wash to the economy as a whole, since most defense spending is spent here on manufacture and R&D of weapons and systems.
But yea the tax would have some negative consequences, thats part of having a tax, and another factor in politicians decisions to use our military might, which they use far too freely right now.