Resending an email of significant importance, whether it is a change in company policy or an extremely enticing sales offer, is not a communication faux pas. Personally, I believe such a strategy, when used sparingly, can greatly help your marketing efforts.
However, if this practice becomes the communication norm for a company, great damage could result. There is a fine line between marketing communications and spam. The best way to circumvent such a debacle is to know your customers.