[PvK]

On point one, see what Loser said. In late-game, it's easy to sweep mines. A vastly cheaper 1-2 ships can glass a planet defended only by mines... unless it has a few Large WP's.

On point two, yes a 40+ ship fleet usually won't be stopped by a planet filled with WP's. But only a few WP's can stop a small group, or as spoon said, slow down a fleet of any size, if it does mobility damage. A single large WP with shield depleters and an ionic disperser has a good chance of stopping an entire fleet in its tracks, and allowing a defense fleet to catch it.

PvK