Again:
Land on ground =/= Fly in atmosphere
I think the possibility to land a ship should be constrainted by physics! If you land a 400 meter-ship on natural ground, it will most-likely not be stable.
I think such big ships should also get structural damage when landing on a surface with the gravity-forces (which are pulling in a different direction than the main thrusters).
At least, just not allowing ships to dive into an atmosphere (by law :D) is crap!!!
And the fact that capital ships donāt have landing gear 
This ^^
Some ships are just made for spaceflight only. The extra mass required to increase the structural rigidity in order to operate capital sized ships within a planetary gravitational environment is not as cost effective as designing/building multiple smaller/atmospheric entry craft to perform the same tasks. I also remember from the old forums Flavien saying cap ships could enter atmospheres, but they will not have landing gears and the fuel cost is quite prohibitive, so it really makes no sense to want to do that. In any event, for I:B, this is a non-issue IMO.
Well, this thread is not about landing (capital) ships on planets. It is about having them collide with planets. If this āplanetā is not a planet but a small moon, a collision can happen for example if some of your navigation instruments fail and donāt warn you.
It started out that way, yes.
Fire support from capital ships in atmospheric combat zones around ground installations?
Fire support for capital ships from ground AA sites?
Bombardment of ground sites?
Getting fighters safely passed long-range ground defences and/or blockades?
Ferrying goods/supplies to/from the ships faster?
I meanā¦ just saying. (Admit it, those reasons are cool.) 
Nope. But if they could land, or if they had reason to fly about in-atmosphere, there would be more motivation for doing something about them being destroyed and falling to the ground. Tangential, but not entirely irrelevant.
Also, the huge psychological impact on the population if you park a Flamberge over their city!
Are you saying that INS should include cities and simulate psychological status of the residents?
Sorry, I couldnāt resist 
Nice to see you popping in @Crayfish.