Halo 1 actually does that. The Covenant Elites have neregy swords that dissipate once you kill them, and the grunts had plasma cannons that would just explode when the grunt was killed. That's why it was so epic when they made those weapons pick-able in Halo 2

(And Tartarus's Gravity Hammer in Halo 3!!)

Still waiting to be able to pick up Hunter's built-in arm weapons though....

Or being too heavy to carry or too complicated to wield with a high hit rate.