If plastic string works just fine for cutting grass and weeds in a weed whacker, it should work equally well in a lawnmower. It would be better for lawnmowers to use string trimmer line to eliminate the risk of bodily injury: for example, severing your foot on the blades or being impaled or otherwise hit by projectiles launched by the lawnmower blades.
So, what would be the point of using a lawnmower in this case instead of just using a weed whacker? The same as it is regardless of what it’s using underneath: an automatically even cut and a wider area of grass being cut at once.
I think the only problem with this design is that string trimmer line needs to constantly be replaced, so it would be less convenient. But this could be mitigated by having a mechanism on top of the lawnmower or at the handlebars that lets you release the lawnmower’s hold on the string at the string trimmer head, feed more string out from the center, and reapply the hold.
Instead of having the line spool located down below at the place where the string is locked in place, have it located up above at the handlebars and use some some mechanism that grabs it inside the tube going from the main part of the lawnmower to the handlebars and pulls it down. Or have the mechanism right on top of the main part of the lawnmower.
The only problem then is seeing when you need more line and when you’ve pushed/pulled down enough. That could be made easier by having a transparent mower deck, made of plastic or acrylic or epoxy resin or something.
