But in order to use a particular arms, you have to prove direct descent in the male line from the armiger?
Think how difficult that is if your surname is Smith or Jones?
(The arms I use as an avatar, were used in the 17th century by one John Garrad, whose line became extinct. I only use the arms, rather than the full achievement! )
If the main line dies out a cadet line takes on the arms (undifferenced). A prime example of this is the Duke of Wellington. He was the younger son and bore a mark of difference on his arms. As he became more famous he dropped the difference and used the undifferenced arms - at the same time as his brother (the rightful heir) was using them. The older brother conveniently died without issue, which allowed the Duke to carry on as if he had no brother.
In the 19th century and into the 20th a 'Name and Arms clause' was quite common. A rich man with arms, but no son, would put the clause in his will. If his duaghter's future husband changed his name to the armiger's, the husband would inherit the arms and usually a large fortune. Some single men, with no children, when contemplating their own mortality would arrange to have a 'Ward'. The ward would be raised as if it were the armiger's own. If it was a girl, then the name and arms clause would appear in the will. If it was a boy, he would be adopted and inherit the arms. There is at least one case of a man changing his name twice after his wife died so as to inherit further arms - and money, of course!
The arms I use as an avatar are mine de jure.
Regards
Chas