This rule, or guideline, was very useful in the days when most adults were barely literate, and it is still a useful guide for words that end in the silent E. For example, fate and fat. Is it fated or fatted? Was the “fatted calf” in the Book of Luke fated to be eaten? If you go off your diet, do you get fater or fatter? How about cut and cute? Do we write, “She is cuter” or “she is cutter”? Do we tell someone to “use the paper cuter” or “use the paper cutter”?
Unfortunately, many ancient scribes, typesetters, and dictionary makers too often either used a double consonant where it was not needed or failed to double the consonant when it was needed. As a result, today the number of words that break the rule is about equal to the number that follow the rule. This is a ridiculous problem for you and me and especially for students.
Take the word necessary. Does that double S follow the stressed vowel? No, it doesn’t.
How about these words? Paraffin. Accuse. Illusion. Immediate. Connect. Surrender. Assist. Attract. English gives us hundreds more words like these that seem not to follow any rule.
So what can be done about it? Since there is no absolute law that covers English spelling and no academy or committee (note the two double consonants) that govern English spelling, do not hesitate to drop an obviously unnecesary consonant when you are writing informally (as I just dropped the superfluous S). But do this only in very informal writing or maybe when you are texting. And be sure to keep your dictionary handy when you are writing something important, such as a term paper or an aplication (application) for a job!