They should be avoided, depending on context of course, for the same reason that medical textbooks don't refer to cocks, tits, bumholes, bollocks and fannies. Note that I didn't say MUST be avoided. Though if you do use pejorative phrases instead of real names for persons who are not in a position to answer back, you may be in danger of saying more about yourself than about them. As with the Daily Mail, for example, calling Miliband "Red Ed", which tells us all we ever need to know about the Daily Mail but precious little about Miliband. However, it's a free country. Though references to bananas or monkeys in the context of black people tend to brand you a racist. Of course, if that's what you want...

It's Steve, by the way. Stephen if you're my mother. You're not, are you?