Notice that Honda puts a magnetic drain plug in the automatic transmission but not the engine oil drain plug. So at least some of the time Honda thinks it makes sense. If you look at a picture of one with a bunch of material on it, then clearly it is keeping that material from circulating in the oil(or transmission fluid) (maybe not all of it all of the time but certainly most of it most of the time). Would the filter have taken the material out of the fluid anyhow? Certainly some of it but maybe not all. The Fit with an AT has a fluid filter for the automatic transmission fluid as well but they still also include the magnet. The magnets are not going to grab every metal, mostly just steel and iron and not at all aluminum. But aluminum is soft compared to steel, so it is probably good if the loose steel bits are sequestered on the magnet, and not gouging little scrapes in the shiny smooth surfaces of the soft aluminum engine parts.
Usually I would expect the free floating iron/steel bits not yet stuck to the magnet, will be washed away by the flow of oil and mostly removed when the oil passes through the oil filter.
The potential down side would be if the bits of steel/iron got magnetized by the magnet and then in the violent chaos of the internal combustion engine some how a portion of them got washed away again from the magnet to circulate once more in the engine oil. If they were magnetized they would tend to stick to the surfaces of any iron or steel parts in the engine. Steel bits stuck to rubbing surfaces by magnetism would tend to cause more wear than if those bits were floating free and easily washed away by circulating oil. Maybe they don't get magnetized that much and don't stick that hard. If so then maybe the down side would not be that bad and the benefit might outweigh the harm.
So, not perfect, but at least in the automatic transmission, even though there is a fluid filter there too, for the AT, Honda seems to think a magnetic drain plug in addition makes sense.

 

I went to O'Riley's and asked them for a magnetic oil drain bolt for my fit. They gave me a m14 1.5 bolt when I did my oil change it wouldn't even fit into the threading so I put my original bolt back with a new crush washer and returned the magnetic bolt. Does anyone know the correct size for the l15a7? And I dont wanna be spending $30+ on a drain bolt from skunk 2 or spoon or anything cuz that's kinda ridiculous

 

Forget all those magnetic bolts/plugs. Go buy a neomydium magnet and just stick it on your existing plug. You can find em online....... If you really want to go all out you can JB Weld it on...but it's not necessary...they will grab on the bolt with great force...........z

Wonder Magnets: http://www.wondermagnet.com/