Someone told me that when skyrim respawns items like weapons can be left behind eventually creating a memory or speed lag, so they says it's best to put dead enemies weapons "in" the body so they will be taken along with the body, did they fix this or is this still necessary?

It helps for some and it doesn't have much impact to others. It's up to you actually.

If you don't mind to do it...there is nothing wrong about it.

The latest 1.9 patch was released on April 2013. There will be no more...

That was the latest patch with memory and stability improvements! (whatever that means)

Bethesda never fixed the problem with equipped weapons properly cleaning up with the NPC. USLEEP fixed that on PC, but on Xbox, the weapons NPCs drop after combat will remain there forever unless picked up ,and if not picked up right away, they can become glitched and lose the ability to be interacted with.

Whether you pick stuff up is up to you. You can play for a very long time before that stuff will build up enough to bloat your save to the point where it becomes unplayable. But yeah, if you want a pristine save, you should pick up weapon drops after a battle and stash them in a corpse.