Full prime on all exterior surfaces like siding and trim prior to top coating them with a quality latex is almost always a great idea. If it were up to me, most exterior paint jobs would have full primer and 2 coats of the highest quality latex, but often price is an overriding factor. The fact is that labor is the single biggest cost on a paint job, so I encourage customers to go with the highest grade paint and a full coat of primer should be considered. Is it absolutely necessary? No but it will add to the longevity of a paint job.
Being in business for as long as we have, I can look back at previous work and see that the exterior residential paint jobs that have lasted 15+ years (including my own home) have 2 things in common. Full primer and high end top coats.
The thickness of the layers of paint on the exterior to your home are a major factor on how good the paint acts as a barrier to the elements and time. When you add a full prime to the package you are building up the thickness or mils of that barrier. In the United States paint film thickness is measured in mils. One mil equals 1/1000 of an inch. A piece of duct tape is about 4 mils thick. Gorilla brand duct tape is about 16 mils thick.
Keep in mind that primer does a lot more then just building up those mils. It seals, bonds and levels out the porosity of the substrate.
There are times when full prime should not be considered. For example paint can reach what is called critical thickness. That is when forces causing the paint to peel away from the siding exceed the bond holding the paint film against the surface. If there are many layers of paint on a older home then removal of those layers should be considered. That is time consuming and expensive but sometimes necessary.