Many of us have done it, "Kate can be a bit bipolar can't she?!" meaning that one day she can be in a good mood, the next she can be in a bad mood or "Jane's very OCD, she's great we could do with more people like her!" because she is very organised and tidy. Whilst these are often said in a very light hearted and jokey manner, the consequences of using psychiatric diagnoses to describe peoples' personalities can be serious.
Firstly, it leads to there being positive and negative mental illnesses. OCD is often seen as a positive mental illness, people are tidy, clean and hardworking. The TV programme 'Obsessive Compulsive Cleaners' adds to the damage, the hoarders are seen as lazy or bad and the people with OCD fix the situation and are seen as good. OCD is not a positive mental illness, it is a mental illness that is as equally distressing and horrific as any other mental illness. It affects the quality of life of the sufferer and if an illness is affecting someone being able to live their life then it is not positive. Schizophrenia is seen as a bad mental illness, sufferers are seen as damaging to society, weird and unacceptable "You're such a schizo" and bipolar is seen as someone who can be very grumpy but you never know where you stand with them, "God, you're so bipolar!" A mental illness does not make a person good or bad, it makes them unwell.
By using mental illnesses to describe an individual's personality it both simplifies mental illness and takes the seriousness away from them. Many people work hard to get mental illness recognised as illness. These are diseases of the brain, not personality traits and by using diagnoses as descriptions it takes the severity away and adds to the misunderstanding around them particularly as the descriptions are often very inaccurate. It also implies that these illnesses are set in stone. If you always describe Dave as the depressive, then to you Dave is always going to be the depressive. If the office make jokes about him being a depressive behind his back then these will probably not stop until Dave leaves. It makes it seem as though recovery is not possible, once you have depression then you always have depression. Jane is described as OCD by work colleagues because she is organised, being organised is a part of who Jane is but OCD is not a part of the sufferer, it is the sufferer's illness and it can be beaten.
Unfortunately mental illness is not rare, there probably is someone in your workplace or your class who has mental illness and by hearing mental health conditions being used in a negative way it makes it harder for people to speak about mental illness in a truthful and helpful way. If everyone is joking about someone being bipolar because of their personality then it makes it very difficult if someone within earshot actually has bipolar. They may feel the need to hide their illness and may feel ashamed about it. The more misunderstood an illness is, the harder it is for a sufferer to be open and honest about it. We should be encouraging helpful conversations about mental health, raising awareness and making sure that people are able to talk about their mental illness if need be because whether you joke about mental illness or not, you probably know someone with a mental illness it's just a case of whether you have opened or closed the door that allows them to speak up.