For the average grower who chooses organics, I don't really think it's simply an issue of chemical residue remaining in the plant once harvested, although that certainly is part of it.
To use my own garden as an example, I grow many organic fruits and vegetables. Cucumbers harvested from my organic garden are larger and sweeter than cucumbers from the grocery store. They are actually incredibly sweet tasting, and by comparison the grocery store cucumbers are virtually flavourless. When it comes to nutritional value, the organic cucumbers have many times the nutritional value that an non-organic cucumber will have. It doesn't seem to matter what it is; if it's organic, is smells and tastes better, and is far better for you.
I think for most, the choice to grow organically is multi-factoral.
For myself, I'm going organic for the following reasons...
Better taste - organically grown cannabis, and any other produce for that matter, is just far more favourable, and has a much more powerful and aromatic scent.
No chemical residue - flushing can only get rid of so much. There is always going to be chemical residue left in the plant no matter what you do to try and minimize it. Some people don't care about this, but to others, it's important to be chemical free. Especially folks like myself with health issues who are very sensitive to chemical salts.
Better smell - organic cannabis produces a much more powerful and pleasing scent.
Simplicity of growing - for soil users anyway. You do the work once, mix up your soil, and then only add water till harvest. No need to flush, no need to mix nutes, easy peasy. I don't want to think about how many hours of my life I've spent mixing nutrients and measuring liquids.
Less chance of burning your plants - organic nutrients are often broken down slowly and released into the soil as needed (although there are some fast release organics). It's a lot harder to burn your plants with organic nutrients. As a novice grower myself, with chemical nutrients, I have always had trouble finding that middle ground between adding too much and burning my plants, and not having enough and ending up with deficiencies.
As I mentioned already - health issues. A lot of people are just not comfortable using chemical nutrients. Because there isn't a lot of research done on cannabis, it's hard to know what the effect chemical nutrients are having on our health, and for me it's just better safe than sorry.
There are other reasons of course, but these were the deciding factors for my recent switch.
Hope that helps you out.