The word 'no' is the strongest word in the English language. Every human being should never lose the option of saying no. 
But... Why is it that we have to not say no when we sometimes want to? Anything from "do you want to come shopping with me" to "do you still find me attractive" could sometimes have No as the answer but we are not actually allowed to use it. Why is it wrong to say No It's not ok for you to visit right now. If someone asks to call, rather than say "No I'm just enjoying this tv programme for the next two hours" we instead make an excuse "I'm sorry Im going to pick up my wife etc soon so let's make it another time"
If I ask a question I expect No might be an answer and I dont want the reason to be a cover for the word No. 
"do you want to look at my holiday photographs?"
NO
Is it more important to save the other persons feelings than to do or say what is the TRUTH. 
Does the recipient really have such a frail emotional strength that if you say "no I don't think your hair looks nice" that they don't just think " oh that's their opinion". 
I reserve the right to say no.