Every gentleman should still own a tux... or at least be willing to rent one if he doesn't go to enough formal events to justify owning one.
One of the little things that impressed me about my husband when we were first dating, was the fact that he owned a tux. We were always the best dressed couple at my sorority's formals.
He still owns a tux, and he buys a new vest or cummerbund and tie every once in a while to keep it up to date.