I can buy realism. But two things would make the game more fun while keeping realism.
1) Make there actually be a deficit to being sieged, some sort of drain on resources. Right now, indoor farming means there's no reason to ever go outside. There needs to be some reason to need to go outside to make sieges interesting.
I think the best solution would be with the addition of mages to a siege, necromancers in specific. If a mage outside could cause troubles inside... a wave of possessions, for instance, causing strain on your resources with no legendary dwarves as a result, or the dead rising inside, or coordinating with the chasm/magma/river civilizations, repopulating them if need be... something to cause trouble inside.
2) When the inside has weakened, there needs to be some way, however costly, of getting troops inside. Consider a European siege of an impregnable fortress. Now consider-- if the siegers succeeded and everyone inside starved, they should have some means of now getting into the castle and taking it for their own. As it is, even if all your dwarves inside died at once, goblins STILL couldn't get in. What's the point of even bothering then?
I'm not asking goblins to be superheros. But bringing ladders, portable bridges, and battering rams with them (trolls count as battering rams in my estimation, but the other two don't exist) would go a long way. The use of such things should be costly if they're under fire while using them... but that's why the inside should be softened up first.
The above two things would go a long way to making it more fun, while keeping realism.