In HR circles, "purple squirrel" is a term that is used to describe the candidate who perfectly fits a multifaceted job description. In theory, the purple squirrel would be up to speed in an instant, providing immediate value in all facets of the role. However, there are many problems with hunting the purple squirrel...
First, you will take a very long time finding such an animal. Months will be spent on the search, interviewing a plethora of rodents just to find that the squirrels in question are run-of-the-mill brown squirrels (or even worse, rats with a tail hairpiece) holding exaggerated resumes. This effort will distract you and your team from other tasks that they might be doing, putting you farther behind.
Next, assume you do find a somewhat purplish-tinted rodent. Your team's desire for the perfect purple squirrel will engender endless debate over whether the shade of purple is too blue or too red or whether the rodent in question is a tree squirrel or a ground squirrel and whether that makes any difference. Decisions on whether to use this rodent will take too long and, quite rightly, many purple squirrels will escape your grasp because you grab at them too late. And, still, all the while, your position will go unfilled and things will fall farther and farther behind.
When you can find a purple squirrel that your team agrees on, you will find that purple squirrels generally understand their rarity and will demand top compensation for their uniqueness. Even if you can find the purple squirrel, you may not have the budget to pay for them. If you do, you can assume that there are others out there also looking for your purple squirrel, ready to whisk him or her away for a few more peanuts.
Now, assume you can and do pay for a purple squirrel. Bring it into your environment and you'll notice that their shade isn't quite the shade you needed anyway - there are enough differences in your environment that you end up training the squirrel to be your shade of purple anyway - the squirrel will not provide the immediate impact that you expect. Plus, to keep the purple squirrel effective, you'll have to feed it a special diet of training to keep its unique skills in shape. If you don't do this, you may find your purple squirrel taking on a decidedly brown shade after a while.
Finally, three months from now, when the environment changes and you need a green squirrel instead of a purple one, you'll find that your purple squirrel, having invested many years in eating the proper food and finding the proper environments to allow him to survive as a purple squirrel, will object to your attempt to dye him green. More insidiously, if you attempt to modify your environment towards a different color, you will find the purple squirrel more likely to sabotage than to support the change.
Seriously, you'd be better off hiring an ordinary brown squirrel who has shown a willingness to be dyed whatever shade of color you need.
In reality, many job descriptions are written in too detailed a fashion, not examining how work could be reassigned among other team members or assuming a candidate needs immediate capabilities in tasks that need to be done sporadically. Employees with unique skill sets decrease your "bus factor". And the managers who look for people with all of these qualifications fail because they either don't want to take the time to train or because they don't recognize the difference between satisficing and optimizing. Ultimately, their projects fall behind or fail, too.
You can avoid purple squirrel hunts by performing regular skills assessment for your team and making cross-training a priority. The latter will also improve team-member engagement by increasing their sense of mastery. If you do end up with a purple squirrel on your team, add one more requirement to your list - that your purple squirrel can teach other squirrels how to be sort of purple. By doing this, you'll reduce the need to hire a purple squirrel the next time.
So, structure your team to reduce the need for purple squirrels. In the end, you'll find that a group of happy, harmonious, flexible, and (most importantly) high-quality but somewhat ordinary rodents will probably fulfill your needs just fine.