If we're talking about morbid instants then you have all the incentive in the world to keep your guys back to block because you want your guys and their guys to die to turn on the instants.
"Instead, you sat with your cards in your hand wishing they could do all the awesome things they told you they could do."
"Unfortunately, barring some crazy Deflection tricks or some weird splicing of the Spellskite ability, the awesome effect that card promises is basically never going to happen, and you're going to remain disappointed. Your opponent is just going to work around having to target that creature, and you just used a billion lines of text to (in essence) write "hexproof" on the card. I feel like these kinds of cards are lies. I hate them."This is how I felt about phyrexian obliterator. I wish it just said unblockable
On a related note, think how awesome Morbid would be in M12, where everyone was trading in order to avoid letting Bloodthirst turn on.