I disagree with this general philosophy. The random quests are often rather crappy. Rather than being a bonus, they detract from the overall quality of the game. And if I have to make up my own story to make those random quest worth while, then that's a failure on the part of Bethesda. It certainly doesn't justify their poor quality.
They aren't (IMHO) of "poor quality" they are just simple jobs. I don't see how they detract from the game. It's a simple Bounty, I'm going there to kill a bandit for the Jarl, it's not like I need the Jarl to tell me why I'm going there and why this bandit is a bastard that needs to die and then the Bandit will tell me why he turned into a Bandit and etc..
In the majority of the time the Randomness fits nice with the quest. They are simple quests with simple objectives that usually doesn't need to be more explanation.
Also (To the OP) it's quite obvious (after a certain time) which quests are Random and which aren't.