As someone who has played a least a dozen Castile games (one of my favorite nations to play), I would argue Enrique is NOT the worst ruler in the game. In fact, it's the combination of Juan I and Enrique that gives Castile it's rough monarch point start. That and the fact that Juan is 39 and could easily live for 20 years or more.
Enrique is not the worst leader because you can make him a general. And he makes, at times, a surprisingly good general. In one of my games, he was a 1/4/0/2 general. I put him at the head of an army and let him run all over North Africa. There's something invigorating about a general with stats like that, early game - especially if you don't care if he dies. Of course, then he got the hardy warrior trait ...
Even if you only get a 0/0/0/1, he's a free siege general. Use him until Juan dies and then you'll get an event where you can replace him if you want (not going to spoiler anyone out). And if you're really lucky, you'll get another event that replaces him with an awesome queen!
So he's really not that bad.
On the other hand, a 0/1/0 Queen regency over a 2 year old minor heir is ... awful. Because I've had that and since EU4 isn't Crusader Kings II, you just have to wait it out until she dies or your heir grows up. There's nothing you can do with that.