All drugs CAN have side effects (including, for example, aspirin, paracetamol and ibuprofen) but that doesn't necessarily mean that they WILL have side effects. (If you took too much notice of the list of possible side effects for paracetamol, for example, you'd never take a single paracetamol tablet. Despite that, millions of people take them every day, and hospitals use them as their first line of pain relief, with hardly anyone ever experiencing side effects).
So it is with the dopamine blockers commonly used to treat psychosis. If you give them to Alf, Bertha, Charlie and Deborah, Alf and Bertha might experience no side effects at all, while Charlie experiences a severe side effect of one form but Deborah just has a mild side effect which takes a completely different form. So it's important for anyone taking dopamine blockers to let their GP know about any side effects, in order that their prescription can be switched to something more suitable if necessary.
Look at it this way: If you know that your brain is already doing weird things (i.e. that you're suffering from psychoses), it makes sense to try a drug that might either stop your brain misbehaving or, at least, to make it behave a bit better. You've got little to lose and possibly quite a lot to gain.