While I wouldn't recommend those cheap tools for working on a car or a bicycle or anything, they should be just fine working with plastic. Such as scribers, small table vices, third hand tools, etc. Sure that 50 cent scriber is going to go dull after about 100 uses, and the 10 dollar scriber will last for 1500 uses. Really think about it though, it's a SCRIBER! Sure having one or two of the nice ones is going to be needed for those high precision jobs there you have to have a specific width line scribed in, but for most things, the cheap scriber is going to work just fine.
Using them as tools for what they APPEAR to be intended for(wrenching on a car) is not safe, but when dealing with plastic? They will work just fine. Especially with most of the tools we are going to be using, we're not going to need to worry about it. Good call on posting the heads up, just everyone needs to keep in mind the application.
Michael