Well, for starters, 200 planetary shields (2000 ship shields as you have noted) are needed to stop a photon for anyone on that particular planet

That, in itself, is reason to put SOME shields on a planet.
The following taken from
http://www2.cajun.net/~hugh/tradewar.html
"Level 5 Citadel: Planetary Shields. These shields will prevent photon torpedos from disabling your quasar cannons. Planetary Shields are actually the same as ship shields, and you must buy them and dump them in the citadel. An invader must destroy all the shields before he can even SEE how many fighters are on the planet, which he will have to destroy next! Planetary shields defend at 22.5:1 odds divided by the attacking ship's combat odds. For example, an ISS needs about 15,000 fighters to destroy 1,000 planetary shields."