I think part of the reason they may have gone to 1/1/1/1 is to stop people from having to figure out how much someone actually moves if they get slid diagonally.
DM: The orc smashes the sledge hammer into your side, sending you sprawling. You get slid 1 square.
Player 1: I already moved 3 squares this turn, one of which was a diagonal. Do I get slid 1 square or 0?
DM: 1, because moving diagonally only counts as 1.5 if you are getting moved by a single source. Now, the wizard waves his arms around and chants something you don't understand, and you feel a force tugging you 3 squares, to this spot here.
Player 1: All 3 of those are diagonal. Do I really get moved 3? I know the orc is not the wizard, but he is an enemy. Do I count his one diagonal?
DM: No, just move 3. Now the second orc charges you.
Player 2: I charge to intercept the orc, shifting Bob 1 square diagonally this way so he can't be attacked.
Player 1: So I get slid 1 diagonal even though I already moved a diagonal, right? Because it's not me moving again?
DM: Yes, just move 1. The third orc is tired of your questions, so he moves to attack you as well.
Player 1: I use my tumbling evasion ability to move here. That's another diagonal. Wait. How many diagonals have I moved already?
Player 2: <sigh> 1. I think.
Player 1: Ah, right. So, wait, does that mean I can't move the diagonal because I already moved myself 1 diagonal this turn?
DM: No, it means that from now on, every freakin' diagonal counts as 1 square. Can I kill you with the orc yet? No, seriously, get me some chips or I'm killing you with this orc.
DM: The orc smashes the sledge hammer into your side, sending you sprawling. You get slid 1 square.
Player 1: I already moved 3 squares this turn, one of which was a diagonal. Do I get slid 1 square or 0?
DM: 1, because moving diagonally only counts as 1.5 if you are getting moved by a single source. Now, the wizard waves his arms around and chants something you don't understand, and you feel a force tugging you 3 squares, to this spot here.
Player 1: All 3 of those are diagonal. Do I really get moved 3? I know the orc is not the wizard, but he is an enemy. Do I count his one diagonal?
DM: No, just move 3. Now the second orc charges you.
Player 2: I charge to intercept the orc, shifting Bob 1 square diagonally this way so he can't be attacked.
Player 1: So I get slid 1 diagonal even though I already moved a diagonal, right? Because it's not me moving again?
DM: Yes, just move 1. The third orc is tired of your questions, so he moves to attack you as well.
Player 1: I use my tumbling evasion ability to move here. That's another diagonal. Wait. How many diagonals have I moved already?
Player 2: <sigh> 1. I think.
Player 1: Ah, right. So, wait, does that mean I can't move the diagonal because I already moved myself 1 diagonal this turn?
DM: No, it means that from now on, every freakin' diagonal counts as 1 square. Can I kill you with the orc yet? No, seriously, get me some chips or I'm killing you with this orc.