When you sell products for a customer, you have to be really careful to make them not-dangerous if used by an idiot. If you fix/build stuff yourself, you get to know the ins and outs of the thing you're working with. So it doesn't matter that much if something may be a little dangerous if misused, you should know what to do with it.
One or two screws missing, it's a whole lot better than buying a new one and wasting raw materials and energy and creating waste.
1. Performing dodgy repair jobs on your own equipment for your own personal use.
2. Performing dodgy repair jobs for unsuspecting people that don't know the difference between a resistor and capacitor and are trusting your skill and judgement.
In instance [1] you're performing your own risk assessment and will bear any consequences. In [2] you're performing a risk assessment that will directly impact upon someone else's safety. Their level of risk acceptance may be far more conservative than your own.
...and if your bad repair sets a house on fire you risk the lives of the people who come to rescue you, and the lives of anyone else in the building, etc.
Just note that everyone has a bad day. This may the day you "forget" all the things you normally know and then accidents happen. I don't want to say that you should never use such things, but it is a good practice - even if you just build them for yourself - to improve them in the long-run to decrease the chance of accidents.
One or two screws missing, it's a whole lot better than buying a new one and wasting raw materials and energy and creating waste.