zoom - How effective are the rear weather sealing rings on lenses?

I am kinda torn between buying a Sigma 18-35mm f1.8, Nikkor 17-55 f2.8 and Sigma 17-50 f2.8 for my D3100 and D7100 cameras. The reason for confusion is that the Nikkor one has a weather sealing ring at the rear whilst the Sigma ones don't? Does it really matter that the Sigma doesn't have a sealing ring? How effective are these rings anyway?

Then there is also another factor to consider with zoom lenses that of sucking in dust simply due to zooming in and out, which means that internal zooming lenses will probably fare better than the ones which extend physically when zooming. So with regards to this, Sigma 18-35 and Nikkor 17-55 seem better but not the Sigma 17-50.

Any help is hugely appreciated! Thanks