Solidworks Multibody vs Assembly Modelling

What are the reasons for using one method over the other?

First, what is a Multibody part file?

A Multibody part consists of multiple solid bodies that are not dynamic.
From definition this technique is suitable for modelling frames, sheetmetal products and fabrications in general.

Secondly, why using this technique?
Because of the following Benefits:

1. One file contains the complete parametric information
2. Trim and extend features are quick and easy it is even automated
3. All dimensions and constrains are controlled on the sketch level
4. One file to update with any required changes.
5. There are no need for multiple files and mates
6. Cut list to reference all the components (quantity, material, length, weight,..etc.)

On the other hand Assembly Modelling is required for the following reasons:

1. Designing a dynamic products where system mechanism needs to be tested.
2. Each component has a unique name or number and is tracked individually.
3. Individual  drawing for each part is required (not available in Multibody part environment)
4. Each component is required to be used as a part of another design.
5. When the files grow large in size and may be hard to manipulate in the multibody part environment. Assembly modelling uses techniques for loading and manipulating large design such as (Large Assembly Mode, Light Weight, Speed-pack and Large Design Review)
6. Relocating components in the assembly are more flexible using the assembly mates.

Fine, so that means that ,all fabrication designs should use Multibody parts technique and all mechanism design products should use Assembly modelling technique.

No, this shouldn’t be a rule, because for example if you are modelling large structural design in which thousands of bodies are needed to complete the design. You will end up with a large single part file which will give you hard time opening, using, and manipulating it and resulting in low performance. In this case we need to use a technique which is a hybrid of the above 2 techniques. An assembly files with a number of multibody parts which will give you the best of both worlds.

The conclusion is that, Solidworks offers more than one technique that can help you complete you product design and it is your decision to select the one that best suit your designs.