why is it that toady seems incapable of adding anything without breaking something else?
Welcome to the wonderful world of computer programming.
The more complex a program gets, the less likely the programmer/s are to find all the odd interactions between the parts. It's impossible for a small group of people, or a single person, to do EVERYTHING you can in a game, in all possible variations. Not to mention that different computers may do things slightly differently.
So you test what you can to see if it does what it's supposed to without noticably messing other things up. Then you send it to the playtesters (us in this case) to experiment with it.
The playtesters are the ones that usually figure out many of the odder bugs. Like that dwarves equip more than one weapon in a single hand. Which isn't always something someone tries to do deliberately to see if it happens. Or that having water on a creature and getting it hot causes it's fat to boil off. People can't think of everything.
Not to mention that some things only mess up in certain cases. So it may have worked without fail, but you didn't find out that a dwarf equipped with a featherwood training axe explodes if they eat a firefly because you either didn't have fireflies on your map, or never let your dwarves starve enough to go after vermin.