It's helpful and needed because if you are not backed up with web developing skills then you will have to study it first before applying it together with marketing. I agree with you, good marketing cannot be produced by simply having a marketing skills but because you are going to use internet, you should have good web development skills.
Learning web dev. is not actually necessary in digital marketing although it will be a plus since you will save yourself money for that . What you just need to learn is digital marketing like social media marketing, email marketing, SEO etc. There are many articles and even free online courses out there it will not be hard . And if you actually enrolled yourself in this course you will see in the prerequisite that a knowledge on web dev is not necessarily needed . Its a different thing but its related to each other .
It will be a good factor if you have knowledge in web developing and there are a lot of tools that has been helpful and good through the web. Digital marketing is a different forte and its about marketing your products but having knowledge and background for both will be good.