In Halo 3 there was a reason to lol You melee'd faster than you lunged, for one, so killing multiple people standing nearby was faster. In Reach though, it takes about the same amount of time for both.
Another reason that sword block should stay removed is even if a sword guy gets the jump on a guy with the shotgun, with sword block the shotgun will always win. Sword block doesn't need to be added to the gameplay, the swords lunge just needs to be decreased a little. It isn't that difficult to defeat a sword. A well placed grenade, some spamming, or many other methods will work.
Also, the sword CAN be blocked multiple times (just friggin happened to me) which makes sword block even dumber.