It’s non-negligible to a lot of devs in a couple of ways. A 0.01 part can be a meaningfully visible particle, laser beam, piece of metal, or wire, etc. If a Roblox avatar were the scale of a person, that would be about 3mm or 1/8" to them. That’s not paper or sheet metal thin, it’s more like cardboard box walls or diamond tread plate thickness. Devs are importing 0.01 and 0.02 things already for laser beams, wires, pencils, projectile trails. There are a lot of cases for which 0.2 is just too thick. I experienced this when making electrical arcs, 0.2 looks good from 100 studs away, but a spark from a few tens of studs away really needs to be about 0.025 thick to look realistic and not like blocks. 0.2 studs to the scale of the character is like 2.5 inches, great for a soda can, but not a pen or straw, or anything you want to look actually thin.
The real issue though is that a 0.01 part has volume, so it has mass and can be physically simulated, accelerated, pushed. Roblox is first and foremost a 3D physics simulation based game engine. Letting parts go to 0.01 doesn’t make them degenerate or cause singularities, or calculations to crash throughout the existing system. Actual zero would have a lot of issues. It would require special case code all over the place to tolerate zero-mass, zero-volume things and not subject them to calculations that assume positive, non-zero physical and geometry properties. This is not a limitation that someone just doesn’t want to deal with, it’s the consequence of properly implementing a Newtonian physics model as the top priority and core component of the engine.