I believe every designer should be adept in photography and writing.
Let me run this scenario by you:
You're a graphic designer with an incredible eye, experience, and passion. Cool, you're great at communicating visually. If design is just a hobby to you, then there’s little need to be good at writing.
But if you want to turn this into a career, you have to sell yourself as a designer, and that can't be done with visuals alone. Being a professional designer entails persuading a client-- who doesn't fully understand the mental and emotional requirements of your profession-- to pay you for services and a product that they may not prioritize.
You must learn how to express your visual communications with words, so that people who are hiring you, and most likely aren't visually gifted, can connect with and ultimately pay you. You must deny the temptation to resort to a “creative’s victimization” mindset in dealing with customers. That is, merely passing of a lack of communication on your part as a client not understanding you as an artist.
Many artist’s fall under this mindset, which is ultimately rooted in arrogance. A lack of responsibility is a lack of responsibility, whether you’re branded a creative or not. A failure to communicate to the client is just as unacceptable as is their failure to communicate their brand’s vision.
Now, if you're going the route of creating products with your designs and selling them, such as t-shirts or paper products, you must learn how to stage your products in a complementing way-- to sell. Good design is tainted if presented poorly.
No one wants to buy something based upon a mock-up; you have to learn to communicate through photography, a medium with which the mass of America is comfortable with retrieving information. After all, Instagram, which now has 200 million active users, is entirely photo-based!
Now, go do it!