Transmute Rock to Mud: why "hip- or chest-deep"?

[Ogrork the Mighty]

But why eliminate the need for mud to rock by giving the same effectiveness to dispel magic? Other than it's a lot more convenient for spellcasters to take the latter rather than the former.

 

[Jack Simth]

But why eliminate the need for mud to rock by giving the same effectiveness to dispel magic? Other than it's a lot more convenient for spellcasters to take the latter rather than the former.

Because Dispel Magic doesn't have quite the same effect on mundane mud, such as that found in many swamps, or can be gotten by getting lots of dirt and water....

Edit:
Also, with Trasmute Mud to Rock is also Permanent - so you can do some fun tricks, like turn the mud to rock, quarry the rock, build things out of it, and Dispel them later (or, using minimum caster level to keep the dispel DC down, make a building that... discourages ... casting Dispel Magic in it's confines.....

 

[kerbarian]

Also, with Trasmute Mud to Rock is also Permanent - so you can do some fun tricks, like turn the mud to rock, quarry the rock, build things out of it, and Dispel them later (or, using minimum caster level to keep the dispel DC down, make a building that... discourages ... casting Dispel Magic in it's confines.....

Oh, that's beautiful .

DM: The stone door in front of you has a large, foreboding sigil carved into it at chest height.
Player 1: I cast detect magic.
DM: The door does have a magical aura -- in fact, the dweomer extends through the walls, floor, and ceiling to encompass the entire chamber. After some study, you determine that it's a moderate transmutation effect.
Player 1: It must be a trap. Or maybe it's concealing something. I'll cast dispel magic.

 

[Jack Simth]

Oh, that's beautiful .

DM: The stone door in front of you has a large, foreboding sigil carved into it at chest height.
Player 1: I cast detect magic.
DM: The door does have a magical aura -- in fact, the dweomer extends through the walls, floor, and ceiling to encompass the entire chamber. After some study, you determine that it's a moderate transmutation effect.
Player 1: It must be a trap. Or maybe it's concealing something. I'll cast dispel magic.

There's quite a few different things you can do with that, for different types of effects.

A king of a rather swampy kingdom wants a castle built. An enemy king secretly has a Druid/Wizard/Sorceror hireling/minion to supply stone made by way of Transmute Mud to Rock, and underbid everyone else. If it ever comes to a war, the wall is easily breached (trivially, by the same hireling/minion).

Another builder needs a solid foundation in a muddy, swampy area; Transmute Mud to Rock is ideal for the purpose, other than that nifty little duration.....

It's great for getting a Cheater of Mystria down:
DM: So you turn on your AMF?
Cleric: Of course! Why wouldn't I?
DM: Domain or not?
Cleric: Used the domain earlier; this is one of my normal spell slots.
DM: The cieling above you and the floor beneath your feet suddenly turns to mud. Make a Reflex save.
Cleric: <rolls> Hah! A 19! That translates to a 25!
DM: Oh, and what's your save DC on your 8th level abjourations?
Cleric: 26. Why?
DM: You get completely coated in mud. Which breaks line of effect for the AMF, and the mud a very short distance away turns back into stone.
DM: Everyone else: There's a crash, and suddenly the priest of Mystria who has done so much for you is a strange, misshapen, lump of rock.....