Note I am not saying a rap has to have structure, though just about all do, just not verse chorus structure, but rather that they can benefit from structure.
At it's most base level there is structure... Un intended though it may be. For example 99% of rap I have ever heard is based around a 4/4 musical rhythm,which is one reason it tends to work well blended with rock. Because of that the phrasing so used by the rappers tend to be within and using that structure. Where things are less structured is in overall song structure as there are many rap songs and hip hop songs that don't use what a traditional songwriter would call a chorus... As I say they tend to use more an AAA structure.
Some truly have little structure unless it is enforced by the loops the backing uses, but honestly, in many occasions I think that is more about lack of understanding that what is being used is strophic form, and that it is a known song form. The oldest. When I say that I mean that is true for both the rapper who looks at it wondering what the fuss is about and the many songwriters who likewise only know verse chorus song form.
True, in critique terms songwriters do tend to want the song broken into sections, but they don't need to be. In many cases it is easier to review, especially when a critiquer is faced with a screed of prose on a page.
Does that ruin the rap? No. It is just a useful way to break it up. Do rappers need to throw out all the learning and ways of working as the rest of Songwriters do? No. Most often I think it is because, like many young writers, they aren't aware of the benefits and don't see how it applies to them.
In some ways it reminds me how many young people look at older people as if they were never young, never took drugs, had sex lives, did dangerous things, got in fights etc, as if somehow these are strictly theirs, as if they invented them. Just recently my niece was busy complaining how she could speak like her mum and myself, but we would have no idea of the words that were commonly used by her friends. So we challenged her. She was shocked that we knew all the words she chose to challenge us with. I am sure she would have found a few, but nonetheless some of the words were certainly in use when I was young, some when my dad was young. As to swear words, most go back hundreds if not a thousand years.
Equally the old often accuse the young of many things they are not. The mistakes a teenager makes now tends to be very similar to the mistakes my grandfather made, no matter how much they try to kid on they didn't have sex before marriage, get drunk or get in fights. That is all as old as the hills and recurring.
We are more alike than we like to admit.
Rap versus other forms of songwriting is an artificial us against them. It us not about the old and young debate but it is similar. Rap is on the surface a "young" form of songwriting. But just like my niece thought she and her friends reinvented the world, they didn't. They just thought they did. Likewise, many songwriters could do with reminding themselves that for much of it, they too were blissfully unaware of things, thinking they too invented the world, until someone pointed out otherwise.
It is however very good to know what rappers want out of a critique, what they consider to be important. That is very useful.
We certainly can all agree we use rhyme and rhythm, similes, metaphors and other constructs of language. Are we really going to kid each other on that we don't have the rest in common too?