I don't see how a katakana-only is something that is positive? Isn't it the point of learning Japanese to learn Japanese words and not borrowed English words?
That being said, there's many words in katakana in the Japanese language that aren't borrowed from english. And even if a lot of them are, they might not be what you think at first glance. I think it's still beneficial to study them.
Borrowed words come from many languages, not just English. In becoming Japanese, the change how they are said, and in addition, meanings are changed as well.
In other words, you're not studying English words written in Japanese. They are Japanese words that just happen to come originally from another language.
