If you omit a letter from this person, explain why. The committee will notice this, and suspect a problem. Avoid letters from scout masters, priests, etc. They don't help. We want a successful scientist.
Few comments irritate the scientists reading the application more.
Most working scientists do science because it is interesting and fun, not to save humanity. You don't have to extensively justify agreeing with the Admissions Committee that science makes an interesting career. But if there is a specific area that deeply interests you, and you know something about it, go ahead and discuss it. True passion is a strong motivator. If there are problems in your application, for instance, you changed jobs frequently, or had to repeat Organic Chemistry, don't ignore it.