I've mentioned this before. This idea means that you would need many, many different kinds of weapons, such as: knives, swords, spears, etc. It would require more in-game graphics, a civilopedia entry (if needed), and a big hassle. Having to maintain every single unit that you produce is painstaking. If weapons were a separate attachment, you would increase the "production list" with all the units/buildings you can build to about twice, or even three times its size. Civilization is a bit complicated as it is. Adding this might increase the complexity of the game.
This is a good idea, if someone can plan out a certain way to make it so that it isn't annoying and a hassle to manage the production screen and the units.