Imagine this. Yesterday you called at two stores to enquire about a new mobile phone. At one store the assistant was courteous, efficient, knowledgeable, but aloof. At the next store, the assistant was equally professional, but you develop a rapport. Let's say the deals are the same.
Should be always stress the benefits to be more persuasive?
Ask most people and you'll get the same response. (I've being asking for years.)
They'll favour positive arguments, stressing the benefits, over negative every time. Surely it's always best to be positive? Not so fast.