Capacity Planning - Flexibility

What of those toilet cubicles which allow for both sexes - they have a little notice on the outside saying "either"?
They represent a different point of view.  If (to take a healthcare example) your staff are trained to do more than one thing, for example to perform both joint surgery and chest surgery, then you can keep them busy all the time regardless of the mix of cases coming to them.  The same could apply to someone with skills in both health and social care - instead of two people coming to the house (the personal assistant to get the person up and washed, the nurse to check blood pressure and administer medication) only one comes with skills to do both.
Does this make sense? It sounds ideal!
Not always.  There are two reasons why not.

many skills need constant refresh.  In order to be an exemplary surgeon in one particular speciality, you need to practice regularly.  If you have too many different things that you are able to do, then you may not practice enough in any one of them.  For example, nurses trained in treatment of adults have a hard time re-training to become children's nurses, simple because when there's an emergency the rule-of-thumb doses they first learnt and remember first are the doses for adults.
people with multiple skills are often/ always more expensive to train and command higher pay

Multi-use toilet cubicles are great where a restaurant only has room for two or three toilets.  Each cubicle takes up more space as it has to contain its own wash basin, brick walls, etc.  But where numbers are limited then flexibility is important.
Emergency Care Practitioners (ECPs) are more expensive than paramedics because they are more flexible.  In their case, the choice is between two ambulancepeople with a full 1.5ton ambulance and one ECP in a car.  Again, with small numbers (1 or two attending the patient), the ECP is cost-effective.
An A&E department has many more staff and many more patients.  There may be a case for a multi-disciplinary team, ie where individual staff are specialists in specific skills rather than generalists, and patients need to be referred to the most appropriate individual.
Workforce design is an art form.  That is, a specialist skill that relies on more than academic study, that relies on a deep understanding; a willingness to look at alternatives and quantify them and pick the best and second best (3 Ps and 2 Ws - Predicted, Possible, Preferred, Wildcard negative, Wildcard Positive); but principally that relies on experience and knowledge.