If you give the custom character a good story, it won't be a custom character anymore. At best you'll have to choose between a number of consistent paths.
And i really don't understand the 'give them class, race, skill etc'. The origin characters have the same features than our pc. Astarion can bite people twice a day, that's all. Almost all written lines of dialog are made for the pc, even if origin characters can say it if you made them speak instead of you.
I don't see any trade off between origin characters and custom ones. Except maybe the first have very personal quests, but all the rest of the world is the PC personal quest.
