Actually, minor timeframes are important too even if you are a long term trader, just because they could help you to get into the position with the best possible risk-reward ratio. Even if you expect that price movement would last for weeks or months, you should pay attention to choosing the right entry point because it allows you either to reduce your risk or to get larger position with the same risk. There are two main approaches to long term trading: some traders just enter the position and set a huge stop to avoid situations when they could be stopped out by short price movement while other traders attempt to “catch” a position with the lowest possible risk. I prefer the second approach since it offers outstanding risk-reward ratio even you will be stopped out and then have to re-enter for several times. Using this approach you will get a position just in the beginning of the new trend that creates excellent profit potential. So, for those who would use the second approach 15-min charts would be also useful.
At the same time, there are no strict rules on what timeframe to use, it all depends on your personal preferrences and the strategy you use.