Yes you are right. In English heraldry, charges have no symbolic meaning. Some do have meaning in the purpose they serve. For instance marks of cadency reveal to which son a coat belongs.

Tinctures have no meaning. There have been times when the names have changed as a fad, eg. during the Renaissance the one popular fad was to replace the tincture names with the names of celestial bodies. Or, a fess gules became Sol, a fess Mars. Precious stones were also used. There are a number of these. I'll post some when I get a chance. There also have been some to say that metals are more important than colors and Or has greater importance than argent. None of this really holds water. A coat's importance is derived from it's owner and the history behind it not the tinctures on it.

Even the so called "stains" have not been proven to be "stains on the honor" of the bearer.

People like to give meaning to most things. A lion is strong, the "king of beasts", etc. And this has made its way into heraldry. Most often, the sybolic meaning is personal. That is when creating a coat, the owner would have had his personal reasons for selecting particular charges and tinctures. All of which would have had meaning to him. This may be based on popular meanings or those only known to the originator.

In the case of the martlet, the popular meaning fortuitously complements the heraldic purpose it serves as a mark of cadency. Also, some charges have become charges because they were symbols for something in real life and just made a natural move to the shield. And there is canting which has its own meaning to be discussed later.

And let's not forget the peddlers of bogus coats of arms. Adding meaning to everything acts as an enticement for the uninitiated to buy a coat which they are not entitled.