I did the tests myself, I really can't spawn a vanilla horse with spawners, the world just crashes.
Yet Grum confirms "Working as intended", is the crash really intentional? If yes, may I ask why?
I downloaded the attached world, the zombie villager with overkill sword needed several strikes to take me down (I had no effects/armor). I then killed him, got his sword and used it against common mobs, they all died in 1 hit.
In short, the bug exists and works, confirmed here.
This attachment is the crash report I got after loading a world with a spawner that spawns a zombie with custom effects, I made this spawner back in 1.4 (if I'm not wrong) and it worked fine until 1.5.2, in fact, I tested it in 1.5.2 a minute before 13w22a and it worked just fine, spawning the mob without troubles and all. However now in 13w22a, it just needs to exist in the world and it crashes on the moment you load the save.
My minecraft is mod-free and totally original, nothing that could be caused by external environment.
Confirmed in 13w39b.
My spawner was a simple zombie one, but they had stone sword and iron plate included, the first zombie spawned had no armor or weapons at all, 2nd zombie and so on were properly equipped though.