So you have a slew of high level wizards and druids (9+ levels). They are tired of adventuring and go into the fortification or architectural business. With the use of rock to mud and mud to rock, the wzards can build a lot of interesting things. Imagine if you will....
Build a framework of wood and fill it full of mud. Cast mud to rock and you have a rock structure. Solid rock.The spell effects two 20 foot cubes worth of stone/mud. (The spell description needs some work I think). That 8000 cubic feet. So one 9th druid could create a solid block of stone 10 feet wide x10 feet deep and 80 feet long - a day. Hire some workers, build a frame, fill it with mud and thence again the next day. One could build a square stone fort with 80 foot walls to a side, 10 feet wide and 20 feet tall in 8 days.
Three a month
36 a year
360 in ten years.
Imagine what ten could do.
Want to move a mountain - go right ahead.
Also, the mud transforms to rock in whatever shape the mud is in. The frame become important. Imagine what one can build with a wood frame. The possibilities are limitless.