For a long time, I really didn’t understand the power of incorporeal. This was because I didn’t think about how much more powerful it becomes as a creature becomes stronger. Low level creatures with incorporeal movement can slip through doors and other thin barriers. The risk of ending a turn inside a solid object and taking a bad damage roll is too serious. Here’s a typical writeup of incorporeal movement:
The breath drinker can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.The Book of Many Things, p. 154
Once a creature has a respectable amount of hit points, ending the turns inside a wall becomes a viable strategy. A d10 of damage, even in the worst case. That is going to be alot less than what higher level PCs can dish out.