The main reason why the characters discussed here cause problems is that various attempts to present them create an illusion of working. When you create an HTML document and either consciously or unconsciously use, for example, the trademark symbol, you will probably see it right on your browser, and so will many others. But a large number of other people will see just a blank, or even have their display messed up by some control function. Although the trademark symbol probably looks somewhat better than the result of using a replacement (like HTML markup <SUP><SMALL>(TM)</SMALL></SUP>, which looks like the following on your current browser: (TM)), the gain is rather small as compared with the damage caused when the vendor-specific method of presenting the symbol does not work.

This is what you may get:

[This gas--argon--is inert.
See pp. 5-11.
Letīs all use UNIX(TM)!
Coeur de filet.] (approx.)

This is what many others get:

[This gas argon is inert.
See pp. 5 11.
Let s all use UNIX !
C ur de filet.]