Don't discount this feature on a fitness tracker. The most recent Apple Watches are better in this respect compared to the first one, but the battery life is still measured in hours, rather than days.
The UX for a Fitbit as a smartwatch is honestly horrible. The Pebble acquihire prioritized watch apps, in (what I see as) a belief that watch apps are necessary to compete with Apple. Resources would be far better spend improving the first-party UX and apps first, before opening up the ecosystem to outside developers. The iPhone dev kit trailed far behind the initial iPhone launch.