This is a topic I think I'm going to get some strong counter arguments on, but it's also something that has been essential for my labs ability to navigate tight funding lines. For my own success, one of the best things I did pre-tenure was diversify my research around two central themes. At any given point we've had two to three minimally overlapping projects around each theme. Some have worked out great and some have resulted in only a single publication. But they have all produced something.
More importantly, this strategy has kept up publications in two different scientific fields. Because of that, we've gotten federal and state funding in each of these areas and continue to seek funding for the different projects in each. Diverse topics means diverse research funding sources and programs.
Of course, diverse research topics also means spreading resources thinner, including time and money. It means having to stay on top of more than one body of literature. You'll find that there are certain things that students can't train other students in. It's time consuming and you risk being the jack of all trades and master of none.
But I'm watching the consequence of a single focus play out with a friend of mine right now. He's been successful as a solid contributor to a field that has ballooned recently. But in the last few years there has been a massive $$ dump into the field, with a focus on a few labs. The result is that those labs have more people and more money and churning out papers rapidly. The field has been suddenly and massively tilted. Not only does this have significant consequences for my friend's research program, but his trainees are on the outside looking in as well.
BTW, neuro peeps, how is that whole BRAIN initiative going to distribute funds?
In any case, I'm not suggesting you diversify your research program in case some funding agency drops a lot of money on your direct competition, but there are numerous benefits to keeping a wide base. It made my pre-tenure experience better and more successful.