Perhaps optimists are preferable for startups, as long as the investors have deep pockets. However, in established businesses sometimes the optimist's estimate

Perhaps optimists are preferable for startups, as long as the investors have deep pockets. However, in established businesses sometimes the optimist's estimate results in an acceptable ROI, but the reality could mean that the business over-invested in the project and the business case no longer holds. It is for this reason that many six month projects are cancelled after a year or so. As is so often the case "it depends".