It's worth keeping clean, structured URL. If Content is same and you're doing proper Redirect after changing the structure, it won't affect your rank most of the time.
Structured website is better for the User + Bot as well it helps both of them to understand website more easier.
Let's take an example.
The web site owner is writing articles about "What is the concept of email marketing?" and putting articles under "/blog/category/marketing/e-mail-marketing/" where sub category is too deep.
They can do something "/blog/marketing/e-mail-marketing/" to avoid deep linking URL and make it more clear. I would suggest to make change "/blog/category-name/sub-category-name/"
Possible URL Structure suggestion for your site is as below
www.example.com/blog/what-is-the-concept-of-email-marketing - Blog Post URL
www.example.com/blog/marketing/e-mail-marketing/ - Category URL