The four styles are tactics that can be used in different situations. The tactic that it is appropriate to use most often is 'negotiate'; the other tactics are used less often and in particular circumstances.
For example, it is appropriate to demand something in an emergency. If you are in a shop and a friend collapses with a heart attack, you would tell someone to phone to call an ambulance. And in those circumstances, it is appropriate for the other person either to concede use of the phone or to make the call themselves (negotiating over payment for the call would be inappropriate). Other people in the shop may want to purchase products, but decide that they should withdraw, to allow the emergency services access: both the shop and the shopper lose out, but with good reason.
This is an extreme example, but each of these tactics can also be used appropriately in normal circumstances. For example, it can be appropriate in the middle of a negotiation to "withdraw", in order to collect your thoughts or reflect on the progress so far. Sometimes you may decide to concede simply because you want to 'choose your battles', and only spend time on those issues that are worth the time and effort.
Inappropriate use of tactics
From the above examples you can see that there are times when it is appropriate to be unassertive. It depends on the circumstances, and it is your choice.
Problems tend to arise, however, when unassertiveness