As all dyes fade with exposure to light, it's logical to presume that they could well change with time. However, unless the useage is very, very critical, probably scientific, then they don't appear to suffer much over decades of use.

It's probably worth remembering that there are general circumstances (filters do not add anything and can detract) and special circumstances (salt water, polarisers, ND graduated filters.)

No one statement covers all eventualities, but at least we have covered the main areas so that the individual can decide.

Or maybe to reduce the cost of the lens as much as possible for a different market segment. Do Pentax save money by doing this? Or is their profit margin actually the same or even less because the unit cost is lower overall?

If your DA-L had its hood it would be the same as the Samsung 18-55mm one. The petal shape allows light into the corners of the image where needed, to avoid vignetting, and the long and short petals correspond to the long and short sides of the image shape.

Long lenses have a smaller field of view and generally need longer, deeper hoods to be effective.

Wide lenses need shorter hoods so they don't obscure the image area. There is more of a compromise the wider the lens gets, as it becomes very difficult to make a hood that shields the lens fully.

A very good "lens hood" is to stand in the shadow of a tree or building.

This can be a designer's assumption, and was until the 1970s when it became fashionable for lenses to be as sharp as possible even wide open. This is why it would be nice if manufacturer's explained what their design parameters were for a perticular lens. It could avoid much disappointment.

It does mean that saying one lens is "better" than another can be a moot point. Better for what? If you want dreamy, soft focus portraits then you probably won't be happy with a sharp lens.

No Patrick, just their right to privacy. We have no right to demand these details.

Why is a fair question.

A hood will serve to physically protect the lens for a start. An impact will likely be caught first by the hood which will avoid most possible causes of damage to the front element. It will shield the front element to some extent from spray and debris as well.

Most importantly, the hood shields the lens from unwanted light and therefore increases contrast and apparent sharpness. It reduces flare.

An ideal lens hood will mask off all the unwanted field of view whilst avoiding vignetting. This is why it's a real advance that these days most Pentax lenses have a dedicated lens hood.

To understand how a lens hood improves the image, stand outside on a bright day and then see how putting our hand above our eyes makes it so much easier to see clearly. This is what a lens hood achieves.

The hood is a very important, if not vital, accessory and happily comes with almost all Pentax lenses except the very lowest cost ones.

Presumably air movement is better than stale air.

As you focus closer, the actual aperture does change. usually lenses would be designed to have a magnification of no more than 1:10. Closer than this and there has to be special design to avoid the change in aperture, which becomes significant.