The short answer is "Yes, but . . . ". The "but" being that most of the leadership development I come across does not achieve any kind of meaningful business impact. The long answer is, "Yes, if the development is properly designed around evidenced-based practices".
So much leadership development is really training, not development (there's a difference). The training is often based on the most recent shiny object that came into an executive's view and HR was told to "get us some of that". Or the company has an annual leadership meeting and something has to be found to fill time on the schedule. Or the organization buys a leadership development program and requires everyone to go through it (to get their money's worth), no matter individual leaders' skill needs These type of programs are seldom, if ever, worth it. They might be interesting and fun but they don't make a difference.
Development that is worth it improves an organization's ability to compete by building the capacity and capability of its talent. Development that is worth it starts from an organization's strategy. A robust strategy should describe the capabilities needed to execute the plan - resources, capital, processes and talent. It should highlight the strategic core of positions and competencies required to drive the strategy. If the strategic core is not clearly spelled out in the planning process, it can be translated from the strategic details. I often work with companies to do that.
Development that is worth it can only happen after the strategic core competencies are defined. Only then can talent be assessed and Intentional Development designed to address gaps or sustain the competencies across levels and time. Intentional Development is not a once a year training event or a program that every leader must attend. Intentional Development is built into a leader's job, is customized to the leader's needs, is focused on achieving a key business result and is supported by coaching, mentoring and feedback. In this sense, Intentional Development is mass-customized.
I might say that another answer to the question about the worth of leadership development is, "Yes, if the development is intentional."
You can also Stop by my Blog to join the conversation on Strategic Talent Management.