I assume you know why those letters are "hard to read", "gray or white" or "low contrast?" It's all
designed to prevent automated systems from adding comments to a blog, since it takes a human being to read the word and pass this step. If you've ever seena comment that looked like an advertisement or a random link to an unrelated site, then you've encountered comment spam. A lot of this is done automatically by software which can't pass the word verification, so enabling this option is a good way to prevent many such unwanted comments.
Unfortunately, this is more about protecting the blog than it is about the reader's convenience. It's the "cost" of doing business in this, the internet age. Could be worse. And probably will be eventually. Enjoy it while you can.