There is no "good" dragon. And they know better then to go "rouge" as they would be expose to unnecessary conflict. Oh, and they are are highly intelligence being, just to throw that out as well. And in respect, they only deal with the Empire and no one else, as the Empire agree to shelter them if the dragons agree to aid them in war.
Like in Redguard. The dragon was used as the last line of offense. The Redguards were being a huge thorn on Tiber's side, so they released that dragon to give them an easy victory. In addition, it also protected some really powerful soulgem, in a very cramped space. To beat it, Cyrus pretty much had to get his sword all toasty from dragon fire. But, then again, Cyrus is a huge exception. Also, in Battlespire, the dragon, although ended up being killed by hoards of daedra, did serve to help the battlemages in training, and protected them from the daedra as best as it could. Also, dragons do speak very eloquent, and are
very loyal to the empire.
Correction. Martin was the one who defeated Dagon and taken the credit as "hero". The PC Hero is nothing more then just a delivery boy throughout the whole game.
Exactly. If the PC tried to fight Mehrunes Dagon, in game, you would get squished by problem. Heck, you cannot kill Mehrunes Dagon at all. And still, the Mehrunes you saw is only his avatar, abet a really big and powerful one. The real Mehrunes is some floating planet somewhere in Oblivion.