They did this to allow card complexity levels to rise at a faster rate. We've seen some cool new card concepts, Defile being one of them. Dreadsteed's pseudo-infinity is and has been an obstacle for Team 5 since Naxxramas when they were going to make it a card but didn't because of Warsong Commander.

You say there is another way around it, making hidden rules for Dreadsteed for the cards Team 5 wants to make, allowing them to coexist. They have confirmed to be doing this for Grim Patron, and most likely Cruel Dinomancer and any other card combo that could break Defile. But there's a difference between Dreadsteed and these cards. Dreadsteed will break it 24/7. The other cards have to be set up and are much harder to break the card, requiring a really awkward change to the card if they chose the same path to fix them as Dreadsteed. Hidden rules aren't healthy for the game, but getting to see them in a game with Defile and Grim Patron or Cruel Dinomancer is a lot less likely than Defile and Dreadsteed because the latter requires 0 setup and can be done by playing the two cards from your hand.

I'm sorry if you like the card, but they aren't nerfing it into oblivion. The main function of the card is still there. You just can't buff it, and depending on how Blizzard's spaghetti code works out, you might even be able to keep the Spiritsinger Umbra combo. This change will allow for even more unique card designs to be put into the game in future sets.

That's all, had to get this semi-rant out seeing all the recent outcry on the forums recently.

Edit: I should clarify, when I say "the main function of a card" I mean what the card does regardless of its deck or interactions with other cards.