The only why that makes any sense is because of the loss of control should you go flat, contrasted with a car where loss of control is less likely. There is no fundamental difference in the tire that would make it less likely to be successfully mended with a quality plug. And, like I said above, worst case scenario (which is you improperly installed a plug, and it somehow flies completely out), you have a hole the size of the plug. It will slowly deflate at speed, not catastrophically. This isn't a great thing, mind you, but it's not going to kill you unless you're utterly unaware of how to ride a bike and ignore the sensation that the rear end is feeling unplanted. Anyone who's ridden on a near-flat tire knows this feeling, it's quite pronounced. If the plug leaks slowly, it's the exact same situation as the hole was to warrant the plug in the first place (slow leak), and this isn't catastrophic either. There are, however, I think three things you need to do right: 1. Use a quality plug kit (Tech Co. makes a very good one). 2. Install it properly. I'd bet my paycheck 99% of the plug failures were due to improper installation, or use of crappy plugs. You can practice it on an old tire, lawn tractor tire, whatever. 3. Do your normal pre-ride check which includes tire-pressure. If the plug is going to fail, it'll be soon after installation. Plug it, do a few short rides to scrub the tire and let it sit. If it's 100% holding, it'll last.