What is heraldry?
The use of coats of arms (sometimes misnamed "family crests") came about when, during the 12th Century, knights began painting devices on their shields which were inherited by their descendants. They were, and still are, a colorful means of identification (rather like graphic "name tags"), of decoration, and of adornment.
The field of heraldry itself has been called a number of things over the years. "Heraldry has been styled 'the science of fools with long memories.'" (John E. Cussans, The Grammar of Heraldry, 1866). It has been called "The floral border in the garden of history." (Ian Moncreiffe and Don Pottinger, Simple Heraldry Cheerfully Illustrated). But at its most basic, heraldry is "designs painted on shields and used to identify the owner." (Kevin Greaves, A Canadian Heraldic Primer), and "Pictorial genealogy." (Noel Currier Briggs and Royston Gambier, Debrett's Guide to Tracing Your Family Tree).
Coats of arms identify individuals and family lineages; there is no such thing as "the arms of Smith" (or some other surname). As one example, in my own family tree there are a number of Warrens. But looking up "Warren" in Burke's General Armory of England, Scotland, Ireland and Wales, we find that there are almost sixty different coats of arms borne by individuals of that surname.
For a more detailed discussion of this issue, we recommend that you see Information Leaflet No. 15, "The right to arms", published by the Society of Genealogists at:
http://www.sog.org.uk/leaflets/arms.pdf
or the answers to the Most Frequently Asked Questions of the rec.heraldry newsgroup, which may be found at:
http://www.heraldica.org/faqs/mfaq
Still, it is a long-standing practice to display the arms of others for
purposes of decoration. Indeed, there are many instances of people
displaying, in addition to
arms to which they may themselves be entitled to bear, if any, arms of
"alliance and allegiance," the arms of their nation or sovereign, the city
in which they
live, their clan chief (if of Scottish or Irish ancestry), the university
they attended or from which they received their degree(s), armigerous
ancestors and other
relations, organizations to which they belong, corporations for whom they
work, and so on.