Even twinks can teach us something, if we know where to look.
So Merlin, the protagonist of Zelazny's second Amber series, and for people who play the DRPG, he is basically the template for a twink. In a setting where most of the characters are either from Amber (with the powers that come with that) or Chaos (with a different set of powers), he's a half-breed with powers from both.
He's hardly alone in this distinction. Mediocre fiction is awash in half-breeds with the best attributes of each side who are, despite their profound badass-ness unprecedentedly rare.
When I see a character like this in a game, my first instinct is to roll my eyes. This is usually a gimmick to try to justify more than average powers or combinations than the rules might normally allow, and that's a big red flag. That may sound cynical, but it's not an unreasonable assumption. However, it does reveal something interesting that might be usable in almost any other game.
See, the truth is that there's a lot of mojo in hybrids. Interesting things don't happen in the middle of things: they happen at the edges and crossroads, and that's equally true of characters. Interesting characters in fiction and play are usually exceptions in some way, and hybrids are a good way to handle that.
First, it's conceptually powerful. White Wolf has pretty decisively demonstrated the power of combining two templates to make a character, and a hybrid character model can tap into that same appeal. The trick is that in White Wolf (or D&D, in the case of Race/Class) is that the combination is the NORM. Every character out there is something from column A and something from column B. There's nothing exceptional about any particular column.
With that in mind, there's something to be said for designing a setting where the combinations are possible, but not the norm. Create the factions and groups and declare that for the most part, they do not overlap, then quietly allow (perhaps even encourage) players to pick more than one. Everyone else may be a musketeer, and that makes it all the more fun when one player used to be a priest (or noble).
Anyway, my point is that while this is a pretty bad habit when used to exploit the system, it also reveals something that can be used to make a game more fun for everyone else.
1 - When I see that a character like this is an NPC, I am tempted to throw the book across the room. A setting has only so many "slots" for interesting exceptions before they start blurring the lines and making such exceptions meaningless. Those slots are for PCs, plain and simple, and every time a supplement author gives one to an NPC, a kitten dies.