1. Unless we're strictly talking about static meshes (such as for scenery), building out the rigging/animation for garbage topologies is going to be absolute hell.
2. Your performance POV was for ONE mesh, but if your quad/triangle count is 10x higher than it needs to be, how well is it going to scale particularly on low-end gaming platforms?
3. Lighting/shadows/reflections can produce some pretty UGLY effects when your surface polygons are a mess.
From a performance pov I've imported ridiculous meshes not intended for games into unreal. It worked.
I can see this being ok for anything other than triple A games right?