I use Canon Cameras and at first, I was also in the same dilemma as you. That is until I went shooting with a client in what I considered heavy rain. He just went ahead popped his camera and started shooting, leaving me find shelter. later he explained what I had witnessed, the benefits of the Weather Seal. However, we must remember, these seals are weather proof and do stop the moisture from getting into the camera, but it is worth taking note, it is equally as important that the camera is also weather proof.

Its made a massive difference to my photography and given me lots of confidence to shoot instantly and not have to wait for the weather to settle or find a sheltered area to take a photo. It is always worth noting, weather sealing is generally a selling benefit and is therefore generally mentioned with the product. If it is not mentioned, I generally take that as no seal.

Regarding the sucking in of dust, during my years I have only found this to be an issue with lenses that are telescopic push lenses. The ones that you have to physically push out to zoom. The Canon 100-400mm L Lens is one such lens. However, this lens does not suffer from dust, but cheaper lens in the past have.

The screw type thread lenses are generally fine; protrude or not. I say fine as some cheaper lenses are just not made well! however, even then, some cheap lenses such as the Canon 50mm 1.8 at £90 although completely plastic, is simply awesome!

To summaries, Weather sealing definitely works with the right type of camera, and dust is mostly an issue with push type zoom lenses. Hope this helps