Under the old rules, prevention and regeneration spells/abilities would be played after combat damage was put on the stack so that people could respond apropately to the damage being dealt.
In an attempt to give this same responsiveness, the block order was established, but the block order doesn't work well enough.
For prevention, the difference between then and now is that the attack can respond to prevention effects by modifying the amount of combat damage he deals. If the goal was to give prevent a chance to respond to damage being dealt, then it needs to know the amount of damage being dealt. but with the block order method that amount can never be know until its too late.
If you prevent 1 damage to your creature in a mult-block or trample situtation, the attacker can now respond by upping the damage dealt to above lethal, or he could even use a pump.
The only way to ensure creature safety is to prevent above and beyond the expected amount of damage, but then this creates waste. If i use a prevent 5 damage spell on my 3/3, my opponent can then respond by only assigning 3 to that creature and the rest to the next blocker or to the player if it had trample. This results in a waste of 2 damage prevention.
IF i play a prevent 3 damage spell on my 3/3, my opponent can chose to assign 6 damage(for agruments sake, i am not giving a set power to the attacker) instead, thus my creature still dies and the prevention amounts to nothing.
In order to save a creature, you have to overcompensate the damage prevention amount. which leads to waste.
If i hold both a prevent 3 and prevent 5 card in my hand and my creature is a 3/3 either i have to make a wasteful play (the 5 prevent) to save it, or if i try to use the less wasteful prevent 3, my creature could still end up dying.
Its a lose lose sitituation because the attacker gets to chose to blocking order, thus he can for the most part assure that either your first creature dies, or you end up wasting a card to save it.
If i have a 2 damage spell and a 5 damage spell, and i want to kill a 1/2, using the 5 damage spell would be a waste of 3 damage.
Prevention spells being played after damage was on the stack could never account for a damage spelled afterwards anyway, so a player using a damage spell with the new system after a prevent spell is played really isn't any different. whats different is the new added option of changing combat damage amounts that makes prevention wasteful.
regeneration was normally played after combat damage was put on the stack.
Lets say my opponent attacks for 5 and i block with a 1/1 and a 1/3 regenerator.
under the old rules, the attacker would usually apply lethal to the 1/1. in order to force us to use mana, the attack assigns 2 to the 1/1 and 3 to the 1/3
lets say i have a prevent 1 spell in my hand that is cheaper than the regeneration cost.
The least wasteful move would be for me to use the prevent 1 instead of the regeneration.
net result is 1/1 dies an 1/3 lives with effiecent mana used.
Now with the new rules:
blocking order is chosen so that the 1/1 is first.
if i play the 1 prevent on the 1/1 and don't regenerate, he assigns 2 and 3 and both mine die
if i play the 1 prevent on the 1/3 and don't regenerate, he assigns 1 and 4 and both mine die
if i only regenerate the 1/3, the kills the 1/1,
If i play the 1 prevent on the 1/1 and regenerate the 1/3, he assigns 2 and 3 and the 1/1 still dies.
overall the result appears to be the same in that under the old and new rules i could only save the 1/3, but with the new rules, in order to save the 1/3 i have to pay the more expensive regeneration cost. Thus the play is less efficent.
However in both these examples the attacker in factoring the regenerative abilities of a creature. However, if regeneration comes from a spell, by the old rules the opponent couldn't really factor in for regeneration when assigning damage, creating the possiblity of a scenario where both creature could be saved. With the new rules, and spell version of regeneration makes no difference because the attack still gets to factor it in with his damage assignment.
Under the old rules, you play prevent once combat is on the stack, if someone plays a removal card in response to your prevention your creature would still die, however the damage directed towards it would go down with it. End result is that a removal is traded for a prevention, and the creature is traded for the damage that could have been aimed else where.
Now with the new rules, if the opponent removes a creature in response to your prevention, not only do you lose the creature, but the attacker gets to readjust the damage assignment too, resulting in a net gain for the opponent and a net lose for you. You still lose the creature, the spell, and you still end up having to deal with that damage.
In both cases, if the removal was played first (with old rules, removal being before damage is on the stack, since there is almost no point in playing removal on a creature with lethal) the prevention spell can still be played to prevent, so there is no change here.