While it is better for page views if you can become an expert in one field, it can get a little stale for the writer. I have found that the writers that pick a particular subject that allows them to diversify into a broader range of topics, serves to keep the writer exploring new possibilities and passionate while maintaining an audience that looks forward to their ideas and opinions.
Nadine May or Sparkster would be perfect examples of what I'm talking about, both of these writers are known for their particular subjects, both are published writers of fiction and non fiction and both tackle new ideas within their particular fields.
Both writers are also very accomplished Hub Page writers with loyal fans but are also able to interest and gain new readers all the time, while staying true to their main topics.
While most writers wouldn't touch the subjects I write about if you paid them, I'm still am able to continuously find new avenues to explore.
I would say find a niche and broaden your horizons from there.