You may grind your shackles here and there, blocks just plain scare me. On top of that shackles do greatly improve your 3rd angle and provide you with greater articulation, where as blocks just move the axle away from the springs for the added lift.

I am not a fan of blocks, but the fact that most lift companies offer lift blocks up to 3" and many of the late model trucks come stock with fairly tall blocks I can't see how they would be dangerous which should be the main concern.  If the cash is available for new springs that is a better way to go, but in my case my custom springs were a little shorter than expected and I already have a long shackle so I am planning to run a short block to even the ride.