@Trekkie Gal
Well, some people decide to dedicate themselves to only one skill such as culinary arts or engineering stuff so they just unlearn the rest of the skills they don't want to use anymore.
A more prosaic reason is that the more skills you know, the longer time-penalty you pay for learning the next skill.
If you've learned skills you don't enjoy using, you may want to unlearn them when new skills are released so that you lower the amount of time it would take to learn the newly released skills.