It’s true; we often know a writer for her skills in one form. Still, plenty of writers have crossed the divide with elegance and mastery. Andre Dubus’ fiction is celebrated. He also wrote essays. Grace Paley, known largely for her fiction, has also published several volumes of poetry. Anton Chekhov wrote both short stories and plays. Raymond Carver’s publications include short stories, essays and poetry. It is possible to do this and to do it well.
Still, most writers pick a side (as the saying goes). Some are simply consumed by one form. Others find their skills don’t translate. Really, only some writers can manage mastery of multiple forms well.
If you’re considering branching out, by all means do so. There’s no harm in a fiction writer experimenting with poetry. In fact, the demands of poetry will no doubt inform your attention to language when you return to fiction. And you may find a comfortable home in poetry, too. That being said, don’t fall into the trap of moving around so much that you never settle long enough to do anything meaningful.