Why is that Dan? Nothing wrong with a crank being ground 0.030" undersize, the crank in my hemi 6 is ground 30 thou under and has been beaten on for 10 years and sees 7000rpm regularly with no issues. I dont belive it weakens the crank by any significant amount. Do you also throw away your blocks once they need more than a +0.020 over bore?
Back when I was doing more engines, the machine shops that we used wouldn't turn down a crankshaft more than 0.020", all I remember is that they talked about weakens the crankshaft, I don't know this as a fact. 0.040" is the most I'll bore a block