By inferior tool, I mean using an inferior knife. There are survivial knives which can be used for digging, rooting etc. without harm, except that you have to sharpen them, which you should be prepared to do of course as it happen with any use of a knife - thus a knife which can't do that is an inferior tool.
Learn how to dig so as to minimize the damage to the knife, this can be done in several ways, for example use the butt / pommell to break up veyr hard top ground which can be packed, then when digging, trail with the edge to minimize impacts. Go a bit slow, working around rocks rather than trying to go through them. Use your hands liberally to scoop out the dirt, the knife is basically just a pick, not a shovel.
There are lots of times when a digging stick is useless, try in very boggy soil, or when the frost sets in. Try to dig through ice with a stick. A decent knife is actually many times faster than something like a small spade because you need a sharp pick to break up the ground.
Of course if you actually have a pick on you, or a pole spur, or mechanical auger, then yeah you use those, but it isn't a "abusive" task for any quality large tool steel blade, those types of steels are actually intended for such work, hence why they are called tool steels.
Now of course on such tools the edges are not left sharp as on knives, so yes the knife can go dull - however if this really is a huge problem for you, I think that in general is a pretty critical issue as part of survival skills should be the ability to restore a worn edge.
So can you do it, yes; should it damage the knife in any way significantly no; do you do it for your first choice; probably not depending on the circumstances (no sticks, frost, lots of roots, etc.).
Of course there are lots of other survival tasks which also put the edge of the knife in similar conditions as digging. For example :
1) cleaning wild game (the hide can be very dirty)
2) cutting thick barked wood (can be encrusted with dirt)
3) getting roots (very good makeshift cordage)
4) cutting very hard bones (nutrition + tools)
5) cutting metals (tools)
A knife which can't dig and thus any of the above things is very limited compared to one that can, and there are lots that can thus the other ones are inferior. But yes, if you happen to have other more specialized tools on you are the time (bonesaw, cheap bypass pruner, metal snips, plasma cutter), then you use them instead of the knife.
-Cliff